Do you have RealMedia file types checked in File>Library>Import Media ?
In my case there seems to be a playback problem with my old and small .rm video files. MC can play them, but playback is jerky and occasionally stops for couple of seconds.
Just before the start of the file playback I can see a text on Playing Now window: "Media Center can not display visualizations for this file type..."
I have the old RealPlayer 8 installed and it can play those files normally.
[size=1
]Media Center Registered 10.0.115 -- C:\Soft\MJB\
Microsoft Windows XP Workstation 5.1 Service Pack 1 (Build 2600)
Intel Pentium 4 2858 MHz MMX / Memory: Total - 1048 MB, Free - 628 MB
Internet Explorer: 6.0.2800.1106 / ComCtl32.dll: 5.82 (xpsp1.020828-1920) / Shlwapi.dll: 6.00.2800.1400 / Shell32.dll: 6.00.2800.1233 (xpsp2.030604-1804) / wnaspi32.dll: Internal ASPI Layer ...[/size]