Unfortunately it could be a lot of things.
I've found XP in general to be very bloated. SP2 fixed some security problems but appears to have added more "crap" too.
Check this out. I have XP service pack 2 on disc. I too do fresh installs from time to time. I use WinCleaner and the Norton Internet Security suite tools to keep windows running smoother. I keep my hard drive defragged.
The other day, I took some of my old parts (AMD 64 3000+ Winchester chip, 512mb ram, older video card, old socket 939 mortherboard) and installed WINDOWS 98 (old copy) on a 5,400 rpm hard drive (6.5 gb) just because I was bored.
At games, of course, it sucks compared to my 3800+ dual core, with 1GB ram and 7,200 rpm 160gb hard drive, 7600GT video card, etc, etc.
But the "old" setup was much faster at loading up Firefox and I.E. 7.0, must faster and loading pages and much faster and downloading over my DSL connection.
Obviously, something was wrong, right?
After spending hours checking, I finally hit upon a security website and went through the steps to shut down unused and unnecessary ports. I also dumped Norton antivirus and Norton's firewall and went instead with trustee, free ZoneAlarm (which doesn't hog resources like Norton).
So, my advice:
1) Hit Control+Alt+Delet, note the processes running in the background. Disable things like Real Player, Apple and Ipod services, etc. that run all the time.
2) Check your system for adware and spyware, which could include some things being downloaded by Microsoft without your notice. Spybot Search and Destroy and Ad-Aware are good programs for finding these. You can download from a variety of sources, but I recommend this link on the Overclockers site, from Sept. 18:
My Freeware Favorites posted by Charlie: http://www.overclockers.com/tips1247/