When you upgraded, did you run the Windows 7 Upgrade Advisor first? It scans your computer and flags up any issues relating to your hardware and software.
Windows 7 Upgrade Advisor - Download - Microsoft Windows
Windows 7 Compatibility: Software Programs & Hardware Devices: Find Updates, Drivers, & Downloads
You can still run it even though you've already upgraded, but going by what you've said so far, it's more than likely your graphics card wants updating for Windows 7.