I cant answer all your questions but heres for some.
You obviously need more space on the drive which has your operating system (C
. So if you dont have anything that is very important on the drive which is next to it (physically) on the HDD (D
then what i would suggest is just formatting D: and then using a program like Powerquest Partition magic to reallocate the space to the C: drive. This can be done without reinstalling the operating system. If you do have stuff on the D: drive and dont want to reformat the drive then you can use the same program to reallocate the empty space on the drive to the C: drive. This is a bit more of a trcky task to perform (by the program) b/c it has to move partition 1 (D
over the amount of space you would want to reallocate to C:.
I always have everythign separated on my pc too. One drive for my operating system (I have 3 installed on separate drives), one drive for my programs and four for my files. Like this i reinstall my main operating system once a year and I dont lose any valuable info b/c everything important is on a separate drive. Plus this is very good for avoiding viruses. Oh, and my operating system drive isnt even my c: drive. Good to place it on a different drive too b/c most programs look for C: