Well, if there were "never ever" any other security tool in that system, something changed that triggered this new behaviour. I mean, this wasn't "always" like this, was it?
A Windows Update could have "updated" displayed drivers (depending on configurations, although I think this wasn't available for XP).
Other possible influences are "theme - related" tools, docking utilities, some little configuration that we even won't think about, updating to latest display drivers, using "default display drivers from Windows"...
Have you tried using the uninstall utility under Windows Safe
http://www.avast.com/uninstall-utility and reinstall from a clean new download of the latest Avast?
I know this last procedure was already asked, but the uninstall utility was not linked to, and no mention of Safe Mode, so please try again and report back (but I can't promise that it will work).
Oh, BTW, when reporting back, please be specific about what exactly you tried, which steps you did and so on.