just install a good virusscanner: example MCAfee/Norton internet security and update your memory to 4 GB
use a good spyware programm: example ad-aware
use a registry programm to delete files what are useless anyway so dont needed: example ccleaner
install microsoft updates (servicepacks will make your pc faster ass well)
service pack 3 is the last new service pack for windows xp
service pack 1 is the last new service pack for windows vista
ps. a anti-virus programm will make your computer slower ass well. so when you have a pc wich is not that fast dont install a heavy anti-virus programm like Norton this will slow down your performence more
try a free one then: avast is a nice anti-virus programm wich dont slow down your pc more