Here is what to do to solve your problem
1) On your XP machine, create a network workgroup called "WORKGROUP"
2) On your XP machine create an account with the exact same name as the user account on your vista machine (MUST BE PASSWORD PROTECTED!) - Allows your Vista machine to see files on your XP machine
3) Create a user account on your Vista Machine with the same name as your XP user acount (MUST BE PASSWORD PROTECTED!) - allows your XP machine to look at your vista machine
Voila! XP and Vista have been networked :-)