Reboot your computer into safe mode and try "Last Known Good Configuration" which will load the last good copy of your registry from tha last time the computer started without problems.
If that doesn't work you can do a system restore from safe mode with command prompt:
1.Restart your computer, and then press F8 during the initial startup to start your computer in Safe Mode with a command prompt.
2.Log on to your computer with an administrator account or with an account that has administrator credentials.
3.Type the following command at a command prompt, and then press ENTER:
4.Follow the instructions that appear on the screen to restore your computer to an earlier state.http://support.microsoft.com/kb/304449/