If you have a big hard drive you could leave Vista as it is, install XP as well and have your pc as dual boot - so when you boot up you choose either XP or Vista.
I am on AOL and when I got a new laptop wi Vista installed, I had problems getting on to AOL. After many conversations and variouss visits to the shops, it transpired ma BT Voyager router was not compatible wi Vista and I had to get a new wireless router. I installed the AOL disc (for Vista) got the router sorted out and it worked.