Let me clarify a little: during an avast! scan, if
1. avast! freezes or crashes, it's probably avast! problem
2. the computer freezes or restarts, it could be e.g. a conflict with some other program, or a hardware problem
3. the computer powers down - it's probably a hardware problem (e.g. overheating).
Generally, if there was a bug in avast! drivers, it might cause bluescreens occassionally - but hardly a power down.