you have to be more specific, do you mean you have a user profile saved on your vista machine that you cant access? Or is this on a company network that uses active directory or something?
If its the former, log on in safe mode (you will can then choose administrator account, then edit you profile or start a new one and see if it works or not. If it does then it is just that profile and you can delete it, if it doesnt you have a bigger problem. If its the latter you have to talk with your network admin