Subject is self explanatory - can anybody help please?

This has only recently started - no problems with other types of video file
(DivX, AVI) etc.

Explorer freezes immediately if in 'Thumnnail' view mode, or when attempting
to open the file or access file properties in the other views.

However the MPG files appear to open OK in other apllications (WMP, WinDVD
etc).