it could be a wide range of things. Anything from a virus to to many programs running at once and hazerdous mailware and progz that get into your comp from the internet.
Some things you can do:
antivirus check
cut down on start up programs
run a bunch of utility progz like pc bug doctor/ad-aware/whatever you can find thats free(not pirated)
maybe a regestry cleaner but i doubt that would help 2 much...
maybe more ram lol