I would download the Avast Uninstall Utility tool from the Avast.com site..
Download aswclear.exe on your desktop
Start Windows in Safe Mode
Open (execute) the uninstall utility
If you installed avast! in a different folder than the default, browse for it. (Note: Be careful! The content of any folder you choose will be deleted!)
Click REMOVE
Restart your computer
Than I would install the new version of Avast 6
If you had any other Anti Virus Software previously before Avast, I would download their Uninstall Utility tool also..
Keep us updated and let us know how things go.