Unable to run Boot-Time scan

I’ve been having serious issues with my dell XPS m1330 laptop running Windows Vista 32 today. It started this morning when I was running a scheduled scan with avast! Free. Durin the scan my laptop began running slowly and eventually windows explorer crashed.

I tried restarting several times, but every time i log in the systwem runs so slowly that it is unusable with windows explorer crashing regularly.

I am able to start the computer in safe mode and it is usable, but on my first attempt to run a full system scan my laptop somehow restarted itself.

Subsequently I have tried to run the boot-time scan from safe mode, but this does not seem to work as it boots straight to Windows.