100% CPU usage

What’s wrong with this?
My computer is clean and Avast is running wild from the startup, this continued even after upgrading to latest version.
The only way to stop it is to unplug the computer from the wall or hard restart, but it rarely doesn’t immediately go to 100%.
I had to take a picture with my cellphone because it freezes everything.
Everything is normal when I disable it by renaming exe file after booting Linux live cd.
Since this is privileged process, I can’t even see what files are being scanned (if any).
I’m running WinXPsp3 on AMD Sempron LE-1200, 2gb ram.

http://i39.tinypic.com/ac79qx.jpg