I would like to know the following information to help focus in the right direction.
a.What is the edition of windows are you running on the computer?
b.Did you make any changes to the computer prior to this issue?
c.When exactly does this issue occur?
I would suggest you to try the following steps.
Step 1: Boot the computer in safe mode:
To use a Safe Boot option, follow these steps:
a. Restart your computer and start pressing the F8 key on your keyboard. On a computer that is configured for booting to multiple operating systems, you can press the F8 key when the Boot Menu appears.
b. Select an option when the Windows Advanced Options menu appears, and then press ENTER.
c. When the Boot menu appears again, and the words "Safe Mode" appear at the bottom, select the installation that you want to start, and then press ENTER.
If you are able to work without any issue in safe mode then you can infer that some third party software is causing the issue
Step 2:Put the computer in clean boot (to figure out if any third party software is hindering the process)
http://support.microsoft.com/kb/929135----> How to troubleshoot a problem by performing a clean boot in Windows Vista or in Windows 7
Perform a clean boot (check step 1 in the link); if everything works fine after a clean boot, you can infer that some third party services are causing the problem.
Proceed with the remaining steps to pin-point out the third party service.
Note: After you figure out the problem causing program, you may have to update or install a higher version of the program, if you seldom use that software you can consider uninstalling
Important: - remember to put the computer back to normal startup follow step 7 in the link.
Step 3: Check with another user profile
If you do not have another user account, you will have to create one.If everything works fine with another user profile, you can infer that the user profile is corrupt, check the link to know how to fix this issue.