Hi,
1. Did you make any changes to the computer before the issue occurred?
I would suggest you to try the steps provided below and check if it helps.
Set your computer in Clean Boot state and check if the same issue occurs.
By setting your system in clean boot state helps in identifying if any third party applications or startup items are causing the issue.
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 clean boot troubleshooting step, follow Step 7 in the provided link to return your computer to Normal startup mode.
If this happen in Microsoft Zoo Tycoon then I would suggest you to refer the link below: http://support.microsoft.com/kb/323941
Hope this information is helpful.