My experience with Avast free version on XP Pro (with updates):
Uninstalled Avast 4 and installed Avast 5.0.377. After logging in, just after the desktop was displayed, PC rebooted, every time. Tried uninstalling (had to use safe mode) and reinstalling, but same behavior. No opportunity to adjust Avast settings.
So I'm afraid I took the easy option and switched to another free AV.
Cheers
John