Today I finally got an Adminpak installed on my Vista laptop!!
Up until today I've been using an additional Windows Virtual PC - which I'll probably keep for now - running on my laptop to do normal day-to-day admin stuff. I have installed on there my Exchange, SQL and other application tools, resource kits and little extras you pick up along the way. Makes it very portable and gets me up and running much faster if I need to rebuild my machine to move to another one.
After find a couple of resources online about using ORCA to modify the installer or simply using the latest adminpak.msi that ships with Windows 2003 SP1 or Windows 2003 R2, I was still stuggling. I guess I just don't have enough developer blood running through my veins 
Today I found Josh's post about installing the adminpak onto Vista RTM. Sadly, I'm still running RC2 
and using the Windows 2003 SP1 version doesn't work. However the adminpak that shipped with the original Windows 2003 build (dated 2 April 2003) installed with no errors but still causing the same symptoms, with ADUC not loading. I registered a couple of DLL's as instructed and it all came together.
Thanks Josh!
Marlon
PS. Attached you can find the batch file you can run to register all DLL's in one go. Simply save as a .CMD file and run with elevated permisions.