Are you sure you have two drives or do you have one drive with two partitions?
If you can't put programs or files on the D drive, then it's probably locked (read only). This most likely means it is a backup partition and should not be messed with, anyway.
The best solution would be to use the control panel's add/remove programs utility first. Go down the list and uninstall all the programs that you don't use, don't need, or don't want. If you don't recognize a program, don't uninstall it. Next, clean your system. If you're running Windows (most likely), then use the built-in disk cleaner by going to Start - Run - and typing "cleanmgr" (without quotes) and hit ENTER. Select drive C and clean everything from it, possibly including the option that says "compress old files". This takes a while and actually slows your performance slightly, but will be worth it if you need the extra space.
Now that you've uninstalled some crap, you should have room on the drive. Download a small utility called CCleaner from here:
http://www.filehippo.com/download_ccleaner/
On the right, in the green menu, click "Download Latest Version"
Once it's installed, find it and run it. In the "Cleaner" section, select everything under the Windows and Applications tabs. If you rarely clean your system, it will clear up a significant amount of space that the built-in windows cleaner wouldn't. Under the Issues section, select everything under Registry Integrity and File Integrity. This will remove many invalid entries in your registry and is safe to use without messing anything up - unlike other registry cleaners. If you uninstalled a lot of programs, then there will probably be a lot of left over registry entries here. This doesn't really free up space, but will make your computer more stable and fast - might as well do it while you're in the program.
Also, in control panel, open Internet Options - General tab - click "Delete Cookies" - Okay then click "Delete Files" - select "Delete all offline content" and click OKAY. This will clear up most of the stuff saved to your computer from the internet, which can free up a significant amount of space if you've surfed the internet alot and never cleaned out these files.
There are a lot of other utilities that can be used. The other ones I have I either paid for or downloaded illegally, so I won't even bring them up. Hopefully by now you have some room left on the drive.
As for the automatic update in Itunes, if this still doesn't work, look at the settings. I don't use it, personally.
Hope this was helpful and not too boring. Good luck.