HELP Computer virus questions.?

I have a dell inspiron 1525 and lately i've been having problems. I was accross the room and no windows were open on my computer but i herd " Congratualtions you've just won a walmart giftcard" So of course i ran over and had no idea where it came from. So i restarted my computer. Other then that episode my computer runs slow at times, videos such on (utube, news clips,abc etc..) are completly slow, sometimes non- exsistence. Its terrible. So i have a virus, spyware, malware?? by the wya i ave no idea what the difference is. PLEASEE HELP. I want to buy a virus software, but dont have the money to spare right now. is there anywhere i can download for free, legally.