Weird, now where did you get a Socket370 board that takes DDR
Try Internet Explorer again. Does it go fast where Firefox goes slow? Or are they both slow?
Also, I would log on as each user, empty their IE Temporary Internet Files, and set the amount to cache to maybe 2MB, as well as raise the Privacy setting to Medium-High. Five zillion weentsy .gif's and .jpg's per user does not make for a nice tidy defragmented hard drive.

Defrag it and see if that helps.
Other than that, a practical suggestion: if there was spyware, then for gosh sakes make all of the regular users into Limited-class accounts and reserve the Admin powers to one Administrator account that's used only when it's needed. Taking some cues from
DetroitSportsFan and
Schadenfroh, if you could run HiJackThis and post the contents of the resulting log file, that could help establish whether there's monkeyshines going on below the surface there. Also run Microsoft Baseline Security Analyzer to look for hidden weaknesses, follow my Secure Is Good! link in my sig for a link to that.
Gotta get a shower and head to work, maybe one of those guys can check out your HJT log and see if it's ok. Good luck
