Same problem here. The FULL scan started yesterday morning and keeps starting up as soon as it finishes or is STOPPED. I was able to get into the settings, kind of tricky since you can't get to the scheduling when scan is running and it starts running within seconds of stopping it. Turned off FULL scan but it still started up again. Rebooted with the FULL scan set to OFF and it did not start up again so I thought the problem sort of was solved but this morning woke up hearing scan complete turned on my monitor and FULL scan was running again.
AVAST free version ver 6.0.100
WinXP SP3
FULL scan was set for 1st of month @ midnite
Scan log shows FULL system scanned on:
3/1/2011 @ 12am (by schedule)
3/29/2011 @12am (right time but 4 days early)
plus about 30 more attempts in past 24 hours
Right now I have the FULL scan on PAUSE