thanks for the tips, though I managed meanwhile, but just to add on your tips:
I always install XP on a freshly formatted drive to be sure :-) So there's no previous software.
The problem might rather be that the freshly installed XP didn't have the security updates (it's SP2). At least the new version of ZA complained about it. Though the older zone I tried first was from 2004 it seems (45.538) (so couldn't ask for newer updates) but maybe it did have a problem with pure SP2...
Anyway, after I removed the older ZA, there might have some traces of it remained...
Operating System:Windows XP Pro
Product Name:ZoneAlarm (Free)