I'm not alone in my opinion on any problems with XP and avast5 (as is clear from the others posts in many forums), whist your seems to be a lone or minority voice. This has nothing to do with good old XP again and again, but your saying there is a problem with XP and avast5, which most certainly isn't the case.
Yes, there are some people experiencing problems with avast5 that happen to be using XP and that doesn't mean their problem is any the less painful and not saying the problem doesn't exist for them; it is a little more complex that because they are using XP and avast5 they will have a problem.
Given the user base of XP I would expect there to be a higher percentage of people posting here with a problem than have Vista or win7 as they represent a larger percentage of the OS user base. Its simple arithmetic, if you have more users then the percentage of those having a problem will have a higher average also.