expand drive C:?

[cliffracer]

My situation is similar to this one, so I figured it would be better to keep it in one thread.

I have two partitions on my hd, C: and D:. C: has my OS and all my data on it and D: is blank. I would like to decrease D:'s size to maybe 20GB and install ubuntu on it. I tried doing this:

You don't need any software for this, Vista has built-in functionality for resizing partitions. Go to control panel -> Administrative Tools -> Computer Management -> Disk Management. Right-click your C: drive and hit "Extend Volume" in the menu that pops up, or if you want to just create a new partition, right-click the unallocated area, click "New Simple Volume" and it'll walk you through it.

but it wouldn't let me extend C: into D:, it would only let me extend D:. I know this isn't exactly the same problem the OP had, but I thought it might work. How can I decrease D: to 20GB and give the rest to C: ?