Hey ContiX,
This is most likely a driver issue. Before going further, run Aero troubleshooter and see what it has to say.
Open the Aero troubleshooter by clicking the Start button, and then clicking Control Panel. In the search box, type Troubleshooter, click Troubleshooting, and then, under Appearance and Personalization, click Display Aero desktop effects. If you're prompted
for an administrator password or confirmation, type the password or provide confirmation.
Refer:
Open the Aero troubleshooterhttp://windows.microsoft.com/en-us/windows7/Open-the-Aero-troubleshooter
Get the latest driver for your graphics card from the Manufacturer’s website. Download and install the one that is released for Windows 7. See if installing it solves the problem.
Check your manufacturer details from DirectX Diagnostic Tool:
1. Open the Direct X Diagnostic Tool. To do this, click Start, type dxdiag in the Start Search box, and then click dxdiag.exe in the Programs list.
2. Click the Display tab.
3. Under Device, look for Name and Manufacturer.
4. Click Exit.
Make a note of the manufacturer, visit the website and download the appropriate driver for your graphics card driver that is compatible with Windows 7.
For details on how to obtain device drivers, see:
Update a driver for hardware that isn't working properlyhttp://windows.microsoft.com/en-us/windows7/Update-a-driver-for-hardware-that-isnt-working-properly
This can also happen because of malware or a virus infection. Run virus scan to remove any trace of such infection and check. Check the post by by Vincenzo Di Russo, MVP on Saturday, May 09, 2009 8:18 AM at**http://social.answers.microsoft.com/Forums/en-US/vistasecurity/thread/ba80504b-61f1-4d71-960f-b561798b7b42**for the same.
Regards,
Shinmila H - Microsoft Support
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