Some people have simulair problems with dos(based) applications in combination with XP and Avast while others can run the same applications just fine. Therfor I believe it is not only Avast/XP that is causing this, but also (as I mentioned earlier) the combination of those two with a certain driver and/or piece of hardware.
Which one? I don't know. There may even be multiple. To research that, one should make a list from all these simulair problems together with the system specs like Everest provides and then look what they have in common when the problem appears with the same application on several systems. Lot of work. This looks to me the way to find out, but since it is so much work, I am not gonna do it
Let's just use the "rem fix" and wait for Avast 4.5 and see if that is solving it.
Thanks to all who have contributed to this thread. I believe it really have teached us something about what is causing this.