I assume that drives C and D are two partitions on the same hard disk.
If so, what are the current sizes at the moment? Are there any other partitions on this hard disk?
You can do this from Vista's Disk Management console, but beware it is full of risks, so make sure all of your data is backed up first and you have a plan to reinstall Vista if things go wrong.
Hidden partitions should not be touched, (for example, hidden recovery partitions).
You cannot increase the size of C without first deleting D and re-partitioning. Think of it like a 'right to left' sequence, eg C: D: E: F: and so on, to adjust C: you first have to change everything from right to left. You cannot move space from E: to C: directly.
The walk-through in the link shows how. It is for Windows 7 but the method is the same for Vista;
http://www.sevenforums.com/tutorials/2670-partition-volume-extend.html
Mark
I have an Acer Aspire 5310 computer, Window Vista operating system. Would anyone know how to increase/decrease sizes of Data (D) and Acer(C) partitions?
many thanks
flt32s

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic