I have got a ASUS 1101HA with Intel Atom Z520 + GMA500 Videochip., powered by Windows 7.
That is a single core Atom CPU. Whenever you see stuttering it means your filters (whichever they may be) are not using your video chip to do the decoding (the GMA500 has full HW video accel. for everything), but are using the CPU instead, obviously killing it.
WMP in Win7 (assuming your directshow filter config is not completely fubar) is using the MS codecs, which implicitly are taking advantage of any hardware acceleration if there is some. Hence the smooth playback when using that player. So check what filters do what on your system, or in MC in particular. MPC Video decoder, ffdshow DXVA decoder - these will take advantage of hardware acceleration for h264 and VC-1 (if you have them installed). Plain ffdshow, coreavc and the likes are all software decoders and they'll kill that CPU for any video bigger than a stamp size.
You have a control panel in Shark's package for what filters decode what. Start tweaking.