If its a compaq, D is probably the "restore" partition. Instead of giving you an actual legal OEM copy of windows they just use the same disc and put it on that drive. if the HDD fails you're SOL and need to find all the drivers and stuff, which is next to impossible with compaq hardware. But that aside, C is really the only place you should be putting stuff, with that setup.

Though the best setup is usually a small C, and big D, and D is for all the data, but compaq does it the other way for the restore drive.