Windows 7 - Desktop Window Manager is disabled

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 troubleshooter
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.