Success!
It turned out to be a codec problem. All was fixed by reinstalling the default codec packs that came with Windows XP.
I'm at a loss to explain why it only effected GWX 1.03 and not 1.00-1.02 or vanilla SHIII 1.04b but it certainly did and I'm glad it's sorted out.
Here's the link explaining how to reinstall the codec packs. It was written pre SP2 so the codec packs are a little different than what's listed but it's simple enough to see what has to be installed once you get to the page.
http://www.free-codecs.com/guides/Ho...ws_XP_2000.htm
BTW, Many thanks to those that replied and tried to help out. Much appreciated!