If you're using the very latest build of v9 you should be fine, though i really don't understand why there are so many people holding on to old versions. It's not like you have to pay anything for an upgrade, so why not use very latest technologies now and not a year or two later when they won't even matter much anymore because there will be another new stuff in the pipeline? Antivirus is only worth much if it's as up to date as possible.
Certain features might not be supported on WinXP, but none that you don't already have with the old version anyway, so updating to the latest won't really change anyting to the worse. It can only be either equal to the old version or even better because certain cloud features work on any system and using old versions also prevents those cloud features to be used. But could be with the latest version. That's why always keeping AV updated to the latest version is the best thing to do.