Having a problem with my File server and connecting to it with a vista computer.

Hi. I'm having problems connecting to my Windows 2000 server with my vista computer. I just recently installed vista ultimate before hand i was using xp pro.
Everytime I try to access my server it just prompts me for a username/password, i didnt set it up to have a password?? I cant seem to fix this problem any suggestions would be helpful thanks.