I am having the same problem running Windows XP pro - SP1
My event viewer shows the following:
Event Type: Error
Event Source: avast!
Event Category: Client
Event ID: 90
Time: 12:46:41 AM
User: NT AUTHORITY\SYSTEM
AAVM - scanning error: x_AavmCheckBootDirectEx: avscanScanReal of *BOOTA failed, 00000005.
This problem has been occuring off and on Since February - sometimes I can log off and then back on and the problem will go away - some times I have to reboot several times - lately nothing will fix it.
Any help is greatly appreciated