Vista allows you to log in locally without using a password. But be default, when accessing the computer of a network, Vista requires that a password be used. If you want to change this so that you can access Vista over a network without using a password, here's what to do:

1. Go to Start -> Run and enter gpedit.msc
2. Go to Computer Configuration / Windows Settings / Security Settings / Local Policies / Security Options
3. Double click on Accounts. There will be an option called Limit local account use of blank passwords to console login only. Disable this option and you will then be able to login to to Vista over the network withotu using a password.