My desktop is about 5 years old… there’s nothing especially WRONG with it, other than being kinda slow and requiring increasingly frequent reboots.
I’m trying to decide whether it’s worthwhile to drop the cash on a new one. What factors into your decision to buy a new computer?
What do I want to do on my computer?
Can my computer do it?
If no, can I upgrade existing components for significantly less than the cost of a new machine?
If yes, will the upgrade extend the usefulness of my computer long enough to justify the upgrade cost?
If not, then I need a new computer.
“requiring increasingly frequent reboots”. Perhaps someone can comment on this. Perhaps the OP may be having software issues rather than hardware issues–and perhaps should reinstall the operating system and other software.
This is probably true-- if I wiped the HD and started over, it would solve part of the problem. But if I’m going to go through all that trouble anyway-- and it is a lot of trouble to re-install a bunch of software, backup all the music, movies, etc-- why not go through the trouble with a new, (much) faster machine?
Top tip: don’t wipe the drive. Replace the drive with a new one then put the old drive in a USB caddy so you can copy everything off it.
The trouble with buying a new PC is that you typically pay for a new copy of Windows at the same time.
Have you run one of the many checkers for bad drivers and DLLs etc? This can often cure reboot issues with almost no pain.
You can also defrag the disk and see if you have crap on your startup menu.
If after this your machine is still too slow, or if you really, really want to use Win 8, then get a new one.
This is one reason you should have a back-up already handy. It’ll cut down on that time used to back-up the files. Reinstalling the OS can be a pain, but even with purchasing a new computer you’re going to have to move files across and install any non-standard software. So it’s not really a huge difference.
One thing which usually speeds up older machines is putting in an SSD:
Low FPS (frames per second) on games.
I keep my machines lean and mean so low FPS means a need a new video card, more RAM, or possibly a new motherboard and CPU.
Outside of state of the art gaming, hardware stopped being an issue for most normal users around 2005 or so. There really isn’t much need to upgrade hardware unless there are specific things you need, like USB 3.0 support, a blue ray drive, or an SSD, and with a desktop you can get those piecemeal instead of buying a whole new computer.
If your computer is slow and requires frequent reboots, I blame Windows. I haven’t used it outside of work for quite a few years, but back in the day I used to reinstall yearly, or at least every other year. Something about Windows just seems to collect cruft until it becomes damn near unusable. So backup all your important data, make a note of which programs you really want to keep and reinstall everything. It will be like getting a new computer, only a lot cheaper. 
If yes to 1) and 2), I would buy a new computer.
I may never buy a new computer. Adjusting to a new version of windows is painful for us old farts and I may never do it again. It is elder abuse.
Why wouldn’t you back up your data and install everything from scratch before you bought a new computer? Then you don’t need to worry about #2 unless #1 fails to fix the problem.
I use this method:
I buy a new computer when I am forced to adopt new technology. This generally means that I buy software that requires more hardware than what I got. Last time it was because I bought Fallout 3 and my video card couldn’t handle it. I figured if I had to replace a video card, it was time to just buy a new computer so I built my own. I am not really a gamer, I just was intrigued by this one game. The last game I bought before that was Riven 
Computers don’t just get slow because they get old. Hardware tends to have catastrophic failure, not a gradual decline in performance. You have software bloat. Have you run cleanup programs like CCleaner and EasyClean? Uninstall programs that you don’t use, then use these to clean your registry. Also make sure you have a good anti-virus program, and a good anti-malware program (I still like Spybot Search & Destroy).
My laptop is 5-1/2 years old and it still does everything I need it to do just like when I bought it. It has seen upgrades from Office 2003 to 2007 to 2010, and XP to W7, and it just keeps humming along so I will keep using it until it stops running. It is 90% used for email, web surfing, Excel and Word, and those things don’t require any more hardware than 6 years ago.
BTW I think this is why PC sales are way down, and the industry is trying to convince us that we should all have tablets. PCs have hit the tail of the S curve.
I work in IT and my home desktop is 6 years old. I keep saying I will get another one ‘next year’ assuming that it won’t meet my needs anymore then but that day hasn’t come yet and may not for a long time.
That said, I agree that your problem is probably software bloat. That causes a gradual slowdown as well as random system failures leading you to believe that you need a new one when all you need is some freeware to make it run like new again.
Download the following programs from download.com and run them:
It will take a while, possibly hours to run them the first time, but you will be left with system that should be like new again. Just follow the suggestions each program gives you and let its do its thing. Make sure you allow them to clean and defrag the registry as well as defragment the hard drive in the process.
If the Dope could get just a small chunk of all the money we save Dopers, they could do away with registration fees.
Or more likely, hookers and blow for the mods.
Your PC should not require frequent reboots. My PCs stay on 24/7 and only get rebooted when absolutely necessary. My last PC was decommissioned just recently after as many years without a rebuild.
Okay, techie stuff coming up:
Run Task Manager in a corner while you do your normal work and note your CPU and memory usage. If your memory usage exceeds the amount you have, then your computer will get slow.
Run the Event Viewer (Start, Run, Eventvwr is one way) and look at the Application and System event logs. Look for lines which have a red ‘Error’ icon. If they’re plentiful, you’ve got a big problem, and report it back here.
Shut your machine down, open it up, and check that the heat sink on the CPU is firmly attached.
I see what you did there. Well done.
My method is:
Is my PC chugging when it is trying to do the simplest thing? Is the operating system older than 5 years? Are the connectors and cables obsolete? Can I afford to buy a new one?
A PC I bought in 2006 recently started to die on me. I bought it fully formed as part of a package deal, and it wasn’t until 3 years later I found out why it was such a bargain - the standards for hard drives had just changed from IDE to PCIe, and the motherboard was all IDE, so as time went on it was harder and harder to upgrade bits. I had bought a PC on the cusp of being obsolete.
When the C drive hiccuped and wouldn’t boot, I attempted a reinstall of WinXP over the top, but accidentally chose the wrong option and wiped the hard drives entirely, losing all my saved files, some of which were not recently backed up. I figured I may be able to rescue them using recovery software, but as they are IDE drives I cannot easily plug them into any other PC I own, so for now, due to lack of funds for setting things up to even make the attempt, I’m a bit lost.