The guide in my sig has the details but essentially you:
1) Find out your GPU using HWInfo32.
2) Get the latest WHQL driver
for Windows 7 from the manufacturer.
3) Uninstall your old drivers
in the control panel and keep a copy of them if you have one. Then reboot into safe mode and run driver sweeper.
4) Reboot back into safe mode and install the newest driver. Use the normal Windows startup if you receive a message saying no video adapter or something similar is found.