I have a Intel 845 motherboard. recently i installed windows 7. when i play games, i get a message that "hardware acceleration is either disabled or not supported by your video card driver". please help.

Click start right click on computer select properties device manager scroll to display adapters if you see a yellow question/exclamation mark or a red X the driver will need to updated/reinstalled
right click select update driver if your computer came with a motherboard disk the drivers could be on it or you might need to download the drivers compatible to run your games or a hardware accelerator driver
you should browse through all of the devices to make sure the drivers have no yellow or red x's on them
http://download.cnet.com/intel-graphics-accelerator-video/3000-2098_4-10163616.html

http://download.cnet.com/nvidia-graphics-driver-windows/3000-2108_4-10630932.html
This is a WHQL release from the Release 260 family of drivers. This driver package supports GeForce 6, 7, 8, 9, 100, 200, 300, and 400-series desktop GPUs as well as ION desktop GPUs. What's new in this version: Adds support for the newly released GeForce GTX 590, GeForce GTX 560 Ti and GeForce GTX 550 Ti GPUsIncreases performance for GeForce 400 Series and 500 Series GPUs in several PC games vs. v266.58 WHQL drivers Adds NVIDIA Update feature

http://download.cnet.com/nvidia-graphics-driver-windows/3000-2108_4-10630939.html
This is a WHQL release from the Release 260 family of drivers. This driver package supports GeForce 6, 7, 8, 9, 100, 200, 300, and 400-series desktop GPUs as well as ION desktop GPUs. What's new in this version: Adds support for the newly released GeForce GTX 590, GeForce GTX 560 Ti and GeForce GTX 550 Ti GPUs Increases performance for GeForce 400 Series and 500 Series GPUs in several PC games vs. v266.58 WHQL drivers Adds NVIDIA Update feature
hope this helps you