Hi,
- Have you made any changes on the computer prior to the issue?
- What is the exact error message received on the computer?
**Method 1:**This issue might occur if the system is infected by virus, So you may perform a Microsoft Safety Scanner to check if the system is free from viruses infection.
http://www.microsoft.com/security/scanner/en-us/default.aspx
**Disclaimer:**If you run the virus scan the program that is infected with virus will get deleted. So you have to reinstall the program. Also if the files and folders are affected with virus then they might even get deleted
**Method 2:**You may also perform a clean boot and check if you are able to exit the program from task manager.
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 Knowledge Base article.
You may also refer the links
http://windows.microsoft.com/en-US/windows7/End-a-process
http://windows.microsoft.com/en-US/windows7/Exit-a-program-that-isnt-responding