Hi,
1. Since how long are you facing this issue?
2. Did you make any changes prior to this issue?
3. Can you able to get to the desktop?
4. Does the error message show up after entering the password?
I would suggest you to check with safe mode and then perform clean boot and check.
Advanced startup options including safe mode
http://windows.microsoft.com/en-US/windows7/Advanced-startup-options-including-safe-mode
How to troubleshoot a problem by performing a clean boot in Windows Vista or in Windows 7
http://support.microsoft.com/kb/929135
Note: After troubleshooting, make sure to put the computer to start as usual as mentioned in Step 7 in the above KB article.