I confirm the issue about Windows Media Player on XP.
It became red and customizing colors (and other parameters) don't work any more.
I can add that uninstalling and rolling back to Media Player 9 the issue is NOT present.
Now, let me make it clear:
1. AVAST should NOT screw the Media Player, it is not "normal" either acceptable.
2. I am not going to replace hardware and OS because of AVAST, no matter if they are "obsolete".
3. I am not going to use another media player because of AVAST, no matter how cool.
4. so please do not say anything along the line of #1 #2 and #3 because that translates automatically in "use another antivirus".
I expect the issue with Media Player to be fixed or reported as "known issue" of AVAST.