I haven't ever used or for that matter seen a tablet system. So I'm making an assumption that the win XP is in some way a cut down (slim) version, so I'm not entirely sure if the OS would be able to have avast installed and work on it.
Are you able to fully back-up your tablet system, so that it could be completely restored (like a disk image), retaining all your data, programs and settings?
If the answer is yes then you could download the trial version and try it. Many of the providers, may however be redundant if you don't use IM, P2P, Internet Mail, etc.
How you use the tablet would dictate it's need for protection. If it is never connected to the Internet and is more of a portable extension of your main PC, then protecting your PC and or network that it connects to would seem more important. If you have a back-up system so you can restore everything easily then that would lower the requirement.
But everything is down to your use and how much risk the tablet is exposed to.