Why Is My Computer Running Slow

I recently purchased Norton Antivirus 2009 and have done several full system scans. It finds threats and gets rid of them yet I still get pop-ups, my computer is as slow as ever and “Trojan Vundo” is constantly attacking my computer and I constantly have to remove it. Whats the point of having Norton if I have to keep removing the same virus? And why is my computer still slow? Can anyone help?

I have cleaned more then half the space i have in my hard drive/CPU and i still have a very very very slow computer can any1 plz help me and tell me step by step what to do? Best answer gets best points