OK, I just want to add something from my point of view. Almost all of you are making an elephant from fly. I mean Security Center is not the main or very useful function of XP. So, if avast really works fine, but SC shows warnings, just do the following:
1. If it reports avast outdate - check computer clock at first and what avast is showing itself.
2. If it reports you don't have antivirus software installed, or it is outdated, but you know everything's ok - just disable antivirus monitoring in SC, and it won't give any warnings about it, even SC remains active and will give notifications about firewall or automatic updates status.
3. Just shut down Security Center service at all (note: Spybot S&D may give you a warning after scan, but in this case you'll be able to add it to it's exclusion list)
I'm writing it because I think it's useless to talk about the same thing again and again. If my memory not fails me, where were more than one thread about Win SC failures. It's Windows, not avast fault. Moreover, as I said, the SC service is almost useless and unimportant point in all Your's XP SP2 installations.
Regards,
Paulius