A lot depends on what CPU is currently installed in your system. Assuming some flavor of Windows, you should be able to get the system to tell you by right-clicking on “My Computer” (simply called “Computer” in newer versions of windows) and selecting “Properties”.
An older version of Windows (such as XP) might say something like:
Pentium(R) 4 CPU 3.60GHz
3.59 GHz, 2.00 GB of RAM
Physical Address Extension
While a newer version (Windows 7 in this case) might say:
Rating: 6.1 Windows Experience Index
Processor: Intel(R) Core(TM)2 Quad CPU Q9650 @ 3.00GHz 3.00 GHz
Installed memory (RAM): 8.00 GB (7.87 GB usable)
System type: 64-bit Operating System
Pen and Touch: No Pen or Touch input is available for this Display
The type of CPU currently in the system will limit what you can replace it with - you can only upgrade within a particular CPU family (other ones aren’t physically compatible). And there may be other limitations - for a system that was never sold with the faster CPU, the system might halt with a message like “Unsupported processor type” at power-on, even if the hardware is capable of running it. Also, a faster CPU may draw more power, requiring upgraded cooling fans and possibly a beefier power supply. It generally won’t be worth doing this on an older system, since you’ll wind up needing / wanting to address the next limitation, and so on. It is often cheaper to upgrade to a new system, or at least a new CPU + motherboard.
It’s mostly a physical thing, not a Windows thing. You’ll have to replace the old processor with one that fits in the same mount and is compatible with your existing motherboard. If you didn’t home-build the computer or have someone do it for you, you may find that you can’t replace the old processor.
My gut feeling is that if your processor is so old that it won’t run the game, then the rest of your computer is either obsolete or close to it. For apps that require cutting-edge graphics (such as games), computers go stale about every 2-3 years.
The only conceivable way to avoid this is to roll your own, hoping that you can salvage as many parts as possible from one rev to the next. You can mitigate the effects of time by buying the top-of-the-line features. These will last the longest. On the other hand, prices fluctuate, so you may be more willing to upgrade more frequently.
Once upon a time, used equipment had some value. Not any more. Unless your computer runs the latest Windows OS, it’s pretty much worthless. In particular, schools won’t accept it as a donation, since they don’t have the resources to support it.
I just wanted to chime in with another opinion. I think that it’s unlikely that your game fails to run because the CPU is too slow (unless you’re getting an error msg specific to the CPU, and even then I’d google the msg to see if something else is causing the problem*). Much more common would be that the game runs on the old CPU, but at an unacceptably slow pace. I think it’s more likely that your graphics are outdated and/or inadequate. Finally, from my own experience, it’s better to save up and buy a new system instead of trying to upgrade your current one.
A classic example is the game Alpha Centauri. At some point players with newer Pentiums began to receive an error message “CPU not supported”. Their CPUs were fine, the program just didn’t realize that. They had to modify an ini file to bypass the CPU check.
Yes, if the game isn’t running there’s some other problem. Install the latest patches for the game, for Windows, for your video card drivers, etc. CPU isn’t usually a limiting factor for games anyway. It’s more video card and RAM.
Btw, if you have a 1.9 GHz Core 2 Duo, you’re plenty ok. If you have a 1.9GHz Pentium 4, then you’ve got crap. Big difference, there.