This tip will work only for XP, Vista and Windows 7. Although you can log in locally without a password, by default, Windows does not allow network users to access the computer without a password. Typically you will receive an Unknown error 31 if this is the case. To change this setting and to allow network access with Blank Passwords follow next steps:
1. Run gpedit.msc
2. Go to Computer Configuration / Windows Settings / Security Settings / Local Policies / Security Options
3. Double click on Accounts: Limit local account use of blank passwords to console login only
4. Disable this option