How to fix the problem of disabled Desktop Windows Manager in Windows 7?

According to your question, it seems that either Windows 7 couldn't properly detect your graphics hardware and thus Aero was not enabled or your GPU is not supported under Windows 7. If you were still using Windows Vista, I would have suggested that your run Windows Upgrade Advisor first before installing Windows 7. But as you've already installed the new operating system, we are out of that option. Anyway, you should check the following before doing anything drastic.

Normally, if Windows Vista could run Aero on your PC, most likely Windows 7 can also run it. But in your case, apparently the built-in WDDM drivers of Windows 7 are not properly detecting your Graphics Processing Unit or GPU. Therefore, you should search for the appropriate graphics driver from manufacturer's website. After updating the driver to its latest version, you should be able to get Aero working on your Windows 7 installation. Since you didn't provide any information about your laptop, it is not possible to state the current version of driver available for your hardware. Most likely your GPU is either from nVidia or ATi (now AMD). So, check their respective websites for driver updates and download the 64 bit version of the latest available driver. Install it on your PC and Aero should be working fine on your PC.