Firstly, thank you Shirik. Good news - managed to get it working, details posted here to help others struggling.
I checked my Control Panel / installed programs list, and had TWO entries for Java - Version 6 Update 13, and Version 6 Update 14 64-bit. Curious - I thought I may have had two installed JREs, so uninstalled the Update 13, but then the MMOUI web page said I had *no* JRE installed. So I removed the Update 14, and tried again.
The Sun download page is particularly unhelpful, and I couldn't even find a link to *just* a 64-bit Sun JRE. I chose the only Windows JRE that I could download, and it promptly installed the x86 / 32-bit version (but it tested OK on their verification link).
I had at this time theMicrosoft Visual C++ 2008 Redistributable Package - but 64 bit edition (to answer your specific question Shirik). Tried downloading and installing MMOUI - still same side-by-side error.
I browsed to the downloaded app and tried running it, and got the same error - which told me that this "side by side" problem is not some cryptic message from the MMOUI authors, but instead a standard Windows error message.
A quick Google later suggested installing the Visual C++ redistributable again. This time, however, I chose to install the *32* bit Visual C++ redistributable ... and ... with breath held ... it worked.
Conclusion - even though I am on 64-bit Vista, I had to install the 32-bit Sun JRE, and the 32-bit Visual C++ redistributable - but this time it did work, for me at least.
Thanks guys - just did my first update in about 6 weeks - and about to log in and see what's happened to my interface
Cheers
Hydarnes