Without a doubt stick with avast, especially since you aren't experiencing any problems or slowdown. However as and when you do switch to XP you will need to uninstall, boot and install avast again to work with the new OS.
If there is a slowdown in XP it may not be avast related as XP requires more RAM, etc. You really need to investigate if your 5 year old system is compatible with XP there may be some hardware that either needs driver updates, etc. Some of your software may also not be compatible with XP.
When I first update from win98se to XP Pro, I ran a MS XP upgrade compatibility program (I'm not sure if this is still available) and that told me things like my scanner and printer weren't compatible and would require driver updates for XP, some other software had to be run in compatibility mode. So the upshot is this is not a simple exercise and you should prepare or it could lead to headaches.