I've got a Dell I bought late '05 - Pentium 4, 1G ram, XP Media Center edition. I forget the HD size, but it's somewhat over 100G. Not sure what started it all, but it was starting to get really slow. Then I had a crash of some sort and was able to restore it from a backup. That got it working (sort of), but things degraded from there. It crashed again a time or two, and I was able to get it halfway running again with "PC Fix-It". Finally, it wouldn't even boot no matter what I tried.
Fortunately, Dell includes a "run home to momma" function whereby you power-up the computer with a certain keyboard sequence and it blows away everything and returns it to the same state it was when you originally opened up the box it came in. It's a pain in the butt, but paid off the second time around after scrapping the first try. Patience is the key. First I did all the "new computer" stuff. Then I uninstalled everything
except the programs I actually use. All the free-ware, bloat-ware, and trial-ware was jettisoned. I then let the thing update itself over the next 3 days - HUNDREDS of updates that just kept coming, as would be expected after a six year time warp.
The upside, however, is that I was blown away at how FAST FAST FAST it was!!! I freaked out the first time I went surfing and ran through the usual programs!
I'd always been religious about running Spy-Bot, CC Cleaner, AVG, etc., and it helped, but never made anywhere near this amount of impact. Even over a year later, it's still faster than I can remember it being for a long time.