optiontips.in-here's my problem

Hello guys, please excuse me if i make any mistakes here,im far from being a comp,pro or wizard ..here's my problem, i own an older COMPAQ PRESARIO model ( sr1010nx) with windows XP...it has slowed down alot & when i restart it i get a bunch of pop up's and have 2 click them off,i was told got 1 or more virus..I was wondering if u guys can tell which anti-virus/spyware programs i can download on the net for free 2 get rid of the viruses?..ive allready downloaded AVG the free edition & its been working good,ive removed 4 viruses ,but i cant get them all,how do i get my computer fully cleaned,like when i take to a tech.??..thank you guys.