Avast Free version scanning stops at 65%, it just doesn’t complete the entire System drive. I have disabled Spybot and any other malware processes, yet it continues to stop at 56%, and the file it gets hung on is different each time.
Wich version have you installed? The 6.0.1203?
Have you try to make a clean install?
Any chance you have a HDD problem in a certain area? Run chkdsk / scandisk (right click the drive → tools → checkdisk), preferably under Windows Safe Mode (much slower but safer if you have HDD surface problems).