C++ runtime error and mpeg4

My main computer always gives me a runtime error and
reports an abnormal program termination and restarts
explorer any time I so much as hover the mouse over a
mpeg4 file. these same files cause no problem at all with
any of my other computers, all are running XP. now I
can't even download anything because the download to
window is pointed at a folder containing a mpeg4 file.
everthing is up to date, and maintained using Norton
Systemworks.