It could be a memory problem, as the rotating disk suggest the video clip is trying to load, but might not be enough memory to load the whole clip. Or Windows is bleeding into the memory area the clip is trying to use. Some clips won't run unless the whole clip is in memory whilst, others will run part of the clip then pause while the rest loads. Try looking on Windows7 forums, I have posted a link below.
http://support.microsoft.com/search/...=10&range=1-10