This is normal when installing Vista from a DVD. It only changes the drive
letters within Vista, but does not change anything as far as your partition
structure goes. If you right click on my computer, select Manage and open
Disk Management, you can see that your partition structure remains as it was.
If you are multibooting with XP, when booting into XP, the drive letters will
be as they were before.
This has caused some confusion when first installing, but it is not a bug
but rather a design feature.