My computer is super slow?

Okah when i first got this computer it use to be fast but now over time its begun to be really slow and idk why? i believe ita bcaz i get on myspace and such which cause viruses so i was wondering what can i do to make my computer faster? And also is there a website i can go and download for free a spyware & virus cleaner thingy? AND ACTUALLY CLEAN EVERYTHING. i dont tink its possible for free but it woodnt help by asking. please and thankyou. =D