I just want to emphasize that I do honestly believe that Avast is at the top of the AV software today. My friends have bad experience with other products (big names, without real strength) and I strongly recommend Avast as the BEST solution.
The possibility can be the registry key that is linked by any mean to the fullscreen mode and Avast is using it for some security repair.
I have 9 private (my own) computers, then few computers at my primary working place, and 22 computers at education institution where I worked by contract. That is the reason I mentioned Avast, cause during last month, after I installed new Avast version, fullscreen capability is off, and when I run the application on computers where other AV product is installed, everything seems to be good.
I just want to say about the possibility, the Avast does few procedures similar to what XP SP2 does, against the registry. Avast is a very good product, and I want it to be even better.