used free avast but no better

Thank you for my earlier assist. I downloaded, installed and did a full scan with Avast Free Download, most current edition.
Restarted and once I got to the desktop, all icons were there, the computer froze.
I had to restart by removing the battery. Then I tried again and I didn't even get to the desktop, just password input and frozen!
Any ideas? It starts in Safe mode no problem. My operating sys is XP.
Whatever I picked up came from a Canadian prescription drug site I was trying to get to.
Thanks so much.