I wish to 'reinstall' Vista

Wife installed Vista, and it really went weird.

Instead of removing XP which was on C drive
It somehow flopped the drive letters so what was c: now is on D: and vice
versa.
so now it looks like both versions are still on the machine but the drive
info is confused.
It works, but very confusing.

So I've backed up to a outboard backup drive and also to DVD's all the stuff
I need to save.

So my question is:
How do I first reformat c and d, and THEN do a clean install of Vista
Premium on C: as it should be.

The issue is that the drive info is presently confused and I want to make
sure that is straightened out first. I know this because if I try to do a
Vista backup of D: which has all the saved files, it's really the Vista
program that gets backup and vice versa. Sort of 'good' twin and 'evil'
twin.

please and thank you.