Er um well yeah. Uninstall it and then reinstall it. Unless you have some reason to believe that XP will simply NEVER recognize Avira as being installed, the most-likely problem is that it installed incorrectly.
Avira ought to be self-aware enough to know that it's going to be installed in an XP environment, and XP is going to want to know if there is an AV software installed, what it is and if it's working or not. So unless Avira is poorly programmed, it's going to have the ability to interface with XP like any other AV software such as Norton, AVG, etc...
The fact that your Avira is not most-likely means that the problem is with Avira.
Did you install it with some other anti-malware software active, such as Spybot. Some of these softwares have "registry protection" features which are a fat load of hooey as they more frequently prevent legitimate reg changes than they prevent illegitimate ones. I'd keep anything that "protects" the registry permanently uninstalled, but at the very least you should have it disabled when installing another AV software, like Avira.
Do some research on Avira's site. If this is a "known issue" with Avira, there should be some documentation on it. If the problem is with XP, my first suspect would be one of these "registry protection" softwares.
