Personally I would wait till windows 10 comes out, but if you really want to do this, I'd just unplug the drive and get another drive to put 8.1 on (plug it in sata 0) then plug the old drive back in another port and copy whatever you need. You'll probably want to grab the whole c:\documents and settings\ folder and any other place you may have stored data. The reason for unplugging the drive is just to avoid accidentally installing to that one instead of the new one.
I would not try to just transplant the entire profile though, as XP and 8 are totally different architectures and it may not work well and even if it does, you could run into issues.