Well…it DOES have the same network card (cause I couldn’t get the onboard LAN on the new mobo to work,) and same CD-RW. And the original HD is still the one that has my OS on it, though it is on it’s second reformat (and therefore third install of XP.) However, I have a new DVD-RW on the way, and plan to install XP on the new HD and then copy all my files over, so soon only the network card will still be original. And if I can get that damn onboard LAN to work, it’s going off to be installed in a Linux box I’m maknig from all those replaced parts. 
Every 3 years or so. As a gamer, this just gets on the edge of painful (I do put in some new parts from time to time, but try not to go overboard), as I wait until I start to get irritated with the old machine (because it is too slow playing newer games, or like for Battlefield 2, simply cannot run it).
I am now questioning that logic, as the computer is one of my main hobbies, and am wondering whether I should always effectively keep an average to above average machine in place (probably through more frequent upgrades).
I should add that one of my problems with more frequent upgrades is that I always have the feeling that I am putting on bandaids - having one top of the line component and everything else as average, rather than ever just having a really fast machine.
Like many others here, I do my chunk by chunk, and it’s always evolving. A single component will typically exist in my tower for about two years before upgrading. Right now I have a 2.8 Ghz P4 and 875 mobo (the newest parts) with the CD and DVD drives, the network & SCSI cards being from the last two computers. The floppy drive is from 3 computers ago!
First computer: used MacSE, 1991
Second computer: 7100/80, in 1996
Third computer: PowerBook “WallStreet” in 1999
(still my current latest and greatest)
5 years + 3 years + 6 years (and counting) / 3 = loosely every 5 years
Since 1999 I’ve had my one and only computer.
HP Pavillion 433 MHz.
Have added 128 MB or RAM to it’s original 64. Now 192 MB.
And a CD burner.
CRT fried last year, so now I have a nice LCD.
Plan to run it till it dies . . shooosh! I know that could bee soon!
I’m another continuous upgrader. Right now I’m using a dual P3-1000 but have a P4-2400 which I bought over 2 years ago as a games machine. The graphics card from the P3 went into the P4 and when I bought a new card, it went back in the P3. I’ve got older machines in service as a firewall and server.
I don’t replace so much as accumulate. I bought my first desktop when I went off to college in 2000. A (650 MHz PIII). I’ve replaced some broken parts and added a few hundred gigs of hard drives. It’s a file server now. I bought a new computer in '02 (A 1.2GHz Duron), and it’s had a few upgrades over the years, most recently a new sound card. It’s a media computer. I bought an iBook in '03, which was replaced at Apple’s expense in '04 because of a faulty logic board that kept causing kernel panics, and it’s the computer I use for day to day stuff.
But, over 5 years I’ve accumulated 3 computers, so about once every 2 years.
That’s how I feel, too. There are times when you may as well just start from scratch (esp. when your OS has gotten to be so unstable that you’re better off wiping the drive completely).
I’ve only owned two machines. One was a Zenith something or other that I bought slightly used from a friend in the late 80s. It was an 8088, I think, and didn’t have a mouse. Lovely amber on black display. Replaced the monitor on it, but otherwise didn’t upgrade or change anything. Well…at some point I acquired an external modem - does that count?
I’ve owned this one since October 1998 (the Zenith died). It’s a very robust IBM Pentium II(?), I think I actually got it right before IIIs came out, 128MB of memory, DVD drive, 400 MHz processor that has served me really well. I’ve been through 2 printers, upgraded to Windows ME, and added an external hard drive so that I could back up the internal one more easily, but otherwise, I haven’t messed with it. I’m starting to look for a laptop because I would really hate to find myself without a computer at home, but this thing seems to be pretty happy still…
GT