First go to the Microsoft Upgrade advisor website to see if there are any incompatibilities with your system and Windows 7 http://www.microsoft.com/windows/com...s/default.aspx
Anything more than 2gb of ram should be removed.
Then try this
Disconnect all unnecessary hardware from the system. This includes hard drives, USB devices, and expansion cards. Remove all but one stick of RAM.
Use the VGA output on your card if possible.
These problems can also be caused by bad RAM. Run a memory diagnostic overnight, using Memest86 or something similar.
If nothing above works, you may have bad installation media. What condition is the disc in? Verify your copy of Windows came from a reputable source. Try burning a new DVD, using the slowest speed possible