In your shoes, I would do another fresh reinstall and then:
1/ update Windows completely (you're on XP service pack 3, right?)
2/ never, I mean NEVER, use Internet Explorer, except maybe to update Windows --> use Firefox instead
3/ never install any 3rd-party ActiveX controls
4/ install Avast, ClamWin, CCleaner, and stop there. No Norton, McAfee, or dodgy unknown Trojan blasters.
5/ no instant messaging. Sorry.
6/ install the absolute minimum of software, and every time you do, make sure that you find and un-check the "start (insert program name here) automatically when Windows starts" checkbox while installing it. don't install any "quick launch" icons either, as those often add background overhead
7/ clean out the "taskbar lint". head down into the bottom right-hand corner, open the program associated with every dodgy icon you see down there (except Avast, Clamwin, and the basic Windows stuff), and disable the program from starting automatically
I run XP respectably on a circa 2005 single-core Dell notebook! And I've only reinstalled twice in the intervening six years.