If you want the full Windows 7 experience the Aero glass interface is a must. Without the slick glass interface a number of key features will be disabled such as taskbar thumbnails and Aero Peek. According to Microsoft the bare minimum requirements for Aero is a graphics card that supports DirectX 9.0 or newer and a native WDDM (Windows Display Driver Model) driver. In most cases when you install Windows 7 Aero will be enabled but on some configurations various issues prevent the feature from starting.
The most common issue I have seen is a lack of compatible WDDM drivers. When Windows 7 was installed compatible drivers were not found on the installation media. In this scenario Windows will search on Windows Update for compatible drivers once an internet connection is established. To make Windows search for new drivers manually just click on the Start Button and type in Windows Update. Then, click Check for updates.
If no WDDM compatible drivers are found for your video adapter on Windows update try looking on the manufacturers Web site.
After you have, or your system has automatically updated your video adpater drivers to compatible WDDM drivers it is best to refresh the Windows Experience Index. This will run the Windows Experience performance tool to benchmark your computer and identify what features should be turned on and off. Click on the Start Button and type in check the windows experience index and hit Enter. Next, click Refresh Now.
If you still have problems try running the Aero Troubleshooting utility by clicking on the Start Button and typing in Find an fix problems with transparency and then hit Enter.
As a last resort you can try the old Windows Vista registry hack to enable Aero glass on low end systems. Let me know if this helps you in the comments below.