The recovery stuff is not usually on D but in a hidden partition. If you get into disk management you should see all the partitions and be able to change their sizes. You might need to delete the D bit first and then expand C to use up the free space. Make sure you copy everything you want off the D drive first.
I did it the other way on my Acer - made the C smaller for the operating system and programs, and made the D bit bigger for all of the data.
In Vista, go to start, control panel, administrative tools, computer management, disk management.