as said above sounds like your registry is clogged up with shite everytime you uninstall a program it leaves shit behind get c cleaner and run the registry cleaner and clean your temp files and defrag the pc. Avast is better than avg as avg 8 is out now so support for 7.5 is going to dwindle. Ive only got 1 gig of ram and my pc flies on xp and ive got a shit load of programs installed take for instance my security
Comodo firewall pro with defense + enabled
Avast 4.7
Spyware Terminator
Spywate blaster
These programs are all free and i visit the darkside quite a bit and never get infrected, dont listen to peeps who say you need paid security cause most times youre paying for the support which dont help mostimes anyway.
Ad adware dont cut it anymore so you may have some spyware but it sounds to me like your registry has loads of shit on it, Xp should fly mate and makes vista look like a donkey so something aint right