I assume you have only 1 drive and it has only 1 partition: C.
If that is so, go to disk management and shrink C, which will leave an amount of unallocated space equal to the amount by which you shrunk C.
Make a partition out of the unallocated space.
Format that space.
You now have C and D partitions. Copy all of your data to D.
Then you can do a clean install to C without affecting your data on D.
Many people would advise you not to then merge C and D back to a single C, but that is personal choice.
Normally, 60 to 80 gigs is enough space for a C partition of Windows 7 and all applications.
If you want to stick with 2 partitions C and D in the long run, try to shrink your current C to 60 or 80 gigs before you make D. Then make a D partition with all of the unallocated space.
If you want to go back to a single C, then just shrink C by enough so that the resultant D can temporarily hold all of your data.
I would back up my bookmarks, emails, and addresses to the new D.
You will have to reinstall all applications if you do a clean install.