You haven't given much information to work on. Are the C and D drives different operating systems or the same type e.g. ME, XP, etc.?
You also require a method of tranfering from one drive to another so that you can alternate which boot up you require (which you have already stated). For this you need a program like partition magic to be installed on both drives as you do not want each drive seeing each other. Once partition magic is installed you can switch between drives.