i was about to post the same question but i found the answer already via Intel's site: Q5: Why doesn’t the Intel 915 Express chipset family support Windows Vista Aero?
The overall graphics architecture and design of the 915 Express chipset family (with Intel® Graphics Media Accelerator 900 graphics engine) was finalized prior to Microsoft releasing details and specifications around the Windows Display Driver Model (WDDM) driver. Given this, there are hardware limitations in the 915 Express chipset architecture that would limit graphics performance and memory capabilities when attempting to run WDDM on Vista.
The hardware limitations of 915/GMA900 graphics center around not having a large enough memory table to support Vista Aero, a key feature that is part of WDDM driver support. This, along with hardware restrictions on surface placement and graphics memory alignment, results in graphics memory usage that is not optimized for Aero performance. Additionally, the lack of native hardware support in the 915 Express chipset family for Vista’s dynamic memory management would result in slower graphics performance and a sub-optimal user experience with Windows Vista Aero.
With these identified limitations, the 915/GMA900-based graphics system would not have met the performance requirements for Aero desktop composition support under Vista. This ultimately led to the decision to begin WDDM support with the Intel 945 Express chipset products, as Intel addressed these hardware limitations with the desktop and mobile Intel 945 Express chipset families.
To provide the best performance and user experience with Windows Vista on Intel 915/GMA900 platforms, Intel will support Windows Vista on 915/GMA900 with the drivers based on the XP driver model. Q8: What level of Windows Vista Aero experience is supported on Intel graphics platforms?
A8: Intel will support the Windows Vista Aero desktop theme beginning with the Intel 945G/GM Express chipset and Intel 965 Express chipset families, as well as with future desktop and mobile chipsets. Graphics - Windows Vista* support frequently asked questions