Vista - Missing drive letter causing big Windows problems

I used partitioning software (Partition Commander) in Vista to adjust the
size between two partitions on HD0. It failed during the operation, however.
I was able to load Windows using their recovery CD and found that the
physical changes hadn't been committed. Unfortunately, it removed the drive
letter from the Vista Windows partition. I originally was dual-booting with
XP on 'C' and Vista on 'Z' (same HD, two partitions). I stopped using XP long
ago so now I'm ready to shrink 'C' down and give 'Z' the extra space.
However, now when I load Vista it no longer assigns drive letter 'Z' to it,
it gets the next sequential fixed drive letter ('G'). As a result I can't
load any profiles or do anything that has a hard coded registry entry (i.e.
z:\windows...). I cannot start the Computer Management console to assign the
drive letter because it points to a registry entry of "z:\windows...". I also
can't change the value in the registry and make all hard coded entries point
to 'G'. I assume it's a Windows Catch 22 - I can't modify the value because
it's in use or because Windows thinks it knows better than me and I'll screw
it up by renaming it. I tried using batch search and replace utilities but
they won't work either (and I am the admin, so it shouldn't be a permissions
issue).

So, how can I assign a drive letter when the function within Vista won't
work or change all the registry entries, especially those specific to
Windows, to use the new drive letter? If the latter route works, then I would
use Vista to permanently assign the drive letter. And I'd use Vista to size
the partitions, since I now know that's a new feature in Windows.