Forgive me if I didn't spend more than 5 minutes looking for a similar thread, there were some that are similar but I hate bringing posts back from the dead

Setup: User A (Admin) and User B (Power User) are concurrently logged into a Windows XP SP3 system. User A (me) is usually on doing something important, but there are times I don't want to close what I'm doing and log off, even though I should, when User B (my wife) wants to check her email.
Issue: While performing a standard diagnostic on my system to make sure it's running lean and mean I noticed that there were multiple events showing the standard access denied code (00000005). Specifically these occurred when User B was logged on. Looking into these it seems that Avast is attempting to scan User A's profile while User B is the active logged on user. Since User B is not an admin the scan is failing as it should (i.e. User B has no access to User A's profile directory). Specifically its failing when scanning User A's Mozilla profile (C:\Documents and Settings\User A\Application Data\Mozilla\....)
All Avast services are set to their default "Local System Account" so I believe that the scanners should be able to access everything on the drive. Is it because FireFox is running and the profile is in use that the scan is failing? Why then wouldn't it fail when User A is the actively logged on user (which it does not error)?
This isn't a show stopper by any means, but certainly something I'd like to know a bit more about and perhaps clear up if I can.
Thanks,
-Kirk