i assume u r using windows/xp.
reduce your start-ups. use win patrol to keep a watch on them.
use tune-up utilities or ace utilities. they clean up your junk files, registry, and also compact your registry. defragment your disk.
trust u use a good anti-virus and anti-spy. avast, avg & ms anti-spy, adaware are good ones in their respective categories.
disable windows services you dont need. here too win patrol; makes your job easy. this will free up resources.
for guidance visit : *mywebpages.comcast.net/SupportCD/OptimizeXP.html