Lately I’ve been having an issue where MediaPortal playback is choppy and suffers from screen tearing. It turns out this happens when the Windows 7 Aero theme gets turned off. I’m not sure why it’s getting turned off sometimes, but at any rate you can force it on through the guide here;
http://www.mydigitallife.info/hack-to-force-enable-aero-in-windows-7/
I’ve applied this on my Media Center. We’ll have to see if it does the job.