Why is my computer still slow?

I have gotten rid of all my viruses. i have defender pro 07 fyi so its trusted anti virus,anti trojan,etc. and it hasnt expired. i dont have much memory taken up and i only have the normal stuff not any games or like that so why is my computer so slow. my dumb azz friend was lookin at on my computer but how coud it affect my comp. if i have anti virus?