blimey, never heard of the avast remover crashing a system before.

have you run scandisk and defrag recently. it could take a while if you've got a large hard drive. starting the computer in safemode will bypass a lot of rubbish that runs in the background of a computer, and will stop a computer freezing during scandisk/defrag (don't forget to switch off your screensaver too).

have you tried formatting the boot record? (go into command prompt and type "format /mbr" without the quotes).