To log on to your system automatically, bypassing the login screen, do the following:

• Click on the Start Button
• Type netplwiz in the search box and then hit Enter (this loads the Advanced User Accounts control panel)
• In the Users tab, uncheck the box next to "Users must enter a user name and password to use this computer."
• Click on the Apply button at the bottom of the User Accounts window
• When the Automatically Log On dialog box appears, enter your password in the two fields where it's asked and then click OK
• Click OK on the User Accounts window to complete the process

After that, when your computer system starts up, Windows Vista will log on automatically.