Problem When Using *.mpg Files, R-Clicking causes explorer.exe error?

If I Right click or rename or even just select any file with a *.mpg extension then my computer makes a thump noise from the speaker & comes up with a box saying "Explorer.exe - Application Error & says the following:-

The instruction at "0x1d1c4dc5" referenced memory at "0x420b0606". The memory could not be "written".

Click on OK to terminate the program
Click On CANCEL to debug the program

If I click on either then all the icons disappear from my desktop for a few seconds & then re-appear! The only noticable difference is that the Less than symbol (<) just to the left of the icons by the clock has disappeared. I studied the event log when windows reappeared & it said-

Type:Information
Date: 24/05/2005
Time: 21:20:15
Source: Winlogon
Category: None
Event: 1002
User: N/A
Computer: MATTS

If I then go to Properties it Says:-
The Shell Stopped unexpectedly & Explorer.exe was restarted.

Any Idea where to go from here. Any suggestions on what mat be going on or how to fix this?

TIA

Matt