What kind of pop-ups?
Could be people spamming you using the "Messenger Service" in XP (it's *nothing* to do with Windows/MSN Messenger).
Try this to see if you still have the Messenger Service running, and, if it is, disable it:
http://www.grc.com/stm/shootthemessenger.htm
If it's Spyware...
Have you tried using AdAware & Spybot S&D in Safe Mode (hold F8 when booting the PC, select Safe Mode, then scan...)
EDIT: Also, get hold of
HiJackThis, do a scan, & post the log here.