From duel boot to single boot?

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I have:

C-XP
D-Vista
E-Storage

Once I've got used to vista and know all my programs work fine with it, I'm looking to get rid of xp, and just use vista. Is it possible to just somehow remove the C partition and add it to the D? And then somehow rearange the drive letters...

Or will I just have to start over again?

 

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Oh that's very annoying. Especially as it takes so long to install things and make sure they work, then you have to do it again!