This is a graphics (video) issue and may be a bit more complicated.
What kind of computer (laptop or desktop)?
What kind of video card is in it? Video RAM?
When you did the change to Windows 7 did you also go to the Microsoft update web page and get all Custom updates from there for it (except for Hardware Updates)?
The list of changes you did from the manual, was it this list?
http://www.mtu.com/upgrades/hoster/windows7-changes.htm