My question was more about the performance rather than virus :P
We use CCleaner and TuneUp Utilities, but still, the PCs gets slow with a couple of months of heavy use (We are a bunch of devs, so no viruzes, downloaders, torrents or crap like that, just IDE tools), so yeah, I'm sorry but I can't believe this part "it runs better than when I bought it and it's NEVER been reinstalled", I would like to but I used the same tools and I am not a newbie with this...
I'm not a dev, so I'm not installing tons of software over and over. It runs Office, Firefox, and just a few other things. Used multiple times a day, but not for something as intensive as development. Defragged automatically every week. I keep at least 50%+ free hard disk space. Runs faster than when I bought it b/c I removed all the HP bloatware that came with it.
EDIT: I guess I should correct myself, this 7 year old computer WAS running XP until last month when I installed Windows 7 RC on it. It ran XP for 7 years without a reinstall and was fast until the day I upgraded to 7. I'm so used to talking about how long it's run XP without issue, that I forgot I had installed 7 on it! Anyway, that doesn't change the fact that it ran so well for so long!