What exactly do you mean by a "default drive"? As a general rule, assuming you have only one operating system installed on the system (please post back if you have more than one operating system on the computer), the "default drive" is the system drive or boot drive on which the operating system is installed. In most cases, this is C:\.
Instead of this question, please explain precisely what you are trying to do or ultimately accomplish. Do you think the "default drive" is incorrect and, if so, why do you believe that? Are you merely trying to change the drive that first appears when you open up Computer (or Windows Explorer) and why? If it is a dual-boot, are you trying to change which operating system is presented as the default and to which the system will boot if you don't change the selection? Are you trying to move some files and folders from one drive to another and thus want that other drive to be the default when accessing those files and folders?
If we understand more about the problem, we can offer better advice that will accomplish what you want to do (which may require different approaches depending on what you mean and what you're trying to achieve). The better the explanation, the more likely the recommendation we make will resolve the issue (and not make things worse because of a misunderstanding).
Thanks and good luck!