I think it's not related - the log record is about scanning BOOT record of floppy A:
How did you check that the boot-time scan was still scheduled after restart - tried to schedule it again?
Please, open regedit and check the value BootExecute in the key
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager
Normally (when nothing is scheduled), only the following line should be there:
autocheck autochk *
When avast! boot-time scanner is scheduled, a reference to aswBoot.exe should appear there.
Is there anything else inside? Is the path to aswBoot.exe short or long?
Can you dump the content of the key to a file and post it here? (note that if you export the key, a big amount of data will be saved - the whole Session Manager key; the only interesting part here is the BootExecute value).