It's totally your choice how to partition your disk and what to use different partitions for if you have more than one.
Windows 7 will work perfectly with only a c:-drive. And it will work just as perfectly and in just the same way with a c: and a d: on the same physical disc.
I just finished formatting my computer and when prompted I selected for Windows Vista / Windows 7 to use the whole HDD. My question is should I have should I have made a different selection as there isn't a small partition for back up or restore points, should I create the partition ? ..Digger