For what OS? If it is Win9x, search your drive for a PWD file and delete all of them. Then, the next time you boot up, type in a username but with no password. You should never be prompted for the username/password dialog box at boot up.
If the OS is Windows 2000, then you will have to goto the USERS AND PASSWORDS control panel applet. There, you will be able to specify the account for Windows to automatically log in with.