Hi Iain,
Yes, but you wouldn't uninstall, you'd either delete the OS files or format
the drive. First, you need to determine which drive is the system drive. If
the Vista Home Premium volume is the system volume, then do *not* format it.
Instead, simply boot into Ultimate, then remove the VHP installation folders
on that volume (may require taking ownership of them). If it is not the
designated system drive, then you can simply format it. Afterwards, run
msconfig and delete the line that loads the unwanted OS installation on the
boot tab.
--
Best of Luck,
Rick Rogers, aka "Nutcase" - Microsoft MVP
Windows help -
www.rickrogers.org