Possibly a security log problem try:
Log in as an Admin User
1. Right Click on My Computer and click on Manage.
2. Under the Computer Managment Window expand System Tools, then expand Event Viewer.
3. Right click on Security and click on Properties.
4. Under "When Maximum log size is reached" select "Overwrite Events as Needed".
Note: You May also want to increase your log size. I recommend it.
5. Enable Fast-User-Switching and the Welcome Screen.
See if this soves your problem.