I'm guessing you have an EDID issue with your graphics card / monitor. When you installed your pal's monitor your system read the EDID of the new monitor and reset the display properties for using it.
It could also be as simple as resetting the refresh rate for your monitor to the proper specs.
You could try going into device manager and uninstalling the monitor then restarting to see if that sets things back to where they were,
Uninstall the monitor and the graphics card (uninstalling all video apps first) then rebooting, let Windows 7 install it;'s generic drivers
, then if all is OK, re install the regular, most recent drivers for both.