Are you able to log on to your user account? Or not?

Attach a screenshot of avast virus chest. If nothing there, then in text file (boot scan) here: (XP) C:\Documents and Settings\All Users\Application Data\Avast Software\Avast\report\aswBoot.txt file

OR

(Vista/Win7) C:\ProgramData\Avast Software\Avast\report\aswBoot.txt

Attach aswBoot.txt so we can see what boot scan found and removed.

http://antivirus.about.com/b/2007/03/11/clean-quarantine-or-delete.htm

[EDIT:]

Run AdwCleaner, Malwarebytes, OTL, aswMBR from here: http://forum.avast.com/index.php?topic=53253.0 Attach resulting logs in next reply as well.