sounds strange.
Make sure you are using the latest drivers for your new graphics card. Check settings in display properties that may effect video playback... (depends on card).
In windows media player try turning on/off various settings in Tools->Options->performance->advanced. You may find the setting that is causing an issue.
If that does not help, go to display properties and turn hardware acceleration down 1 touch.
Also - go to your decoder settings (depending on what video compression you used in Mmoviemaker) and turn off post-processing options. (eg in Xvid).
Theres a lot of settings to try - but there are a lot of things it could be. Try and get it looking good in media player before you worry about Mamewah.