RAM isn't too expensive now-a-days, so it'll be worth upgrading.
In the meantime, If you have a pendrive, stick that into your USB port and use it as a 'ReadyBoost' memory for Vista to cache to.
2GB is recommended in this case ... I use 4GB drive and Vista flies.
XP, I believe, cannot be rolled back to, unlike one could do with XP to, say, Win98.