(and please don't say stupidities because I've never seen a xp theme causing a display problem!!!! and this is not a display problem but it's a bug!).
I bet that I have seen much more than you in the 25 years I am working with computers, and having 2 IT stores
Don't call people or things stupid just because you don't have enough knowledge about things.
Since I never heard anyone else with this problem (at least it wasn't caused by avast), it is likely your system that is one way or another in error. If you like it or not, that is a fact.
You also have clearly told that it happens when doing things while avast is openend. This means it is not only avast doing this.
Already checked for newer display drivers?
Already checked the XP's log files?
Already checked avast's log file?
Already created a list of running applications/processes when this problem appears? < good way to find a course.
Disabled all not essential applications/processes to see if it makes a difference?
The catalyst manager is known to give problems. I suggest you remove/disable it and see if it makes a difference.