You can mirror your C drive onto another clean, formatted hard drive.
If you are using SATA drives, install the new drive, mirror the C drive over, then go to the BIOs to re-assign the new drive as the first boot device. Then you can clear the (previous) C partition on the old drive. Drive letters will automatically re-assign.
If you're using ATA drives, put the new drive in the slave position first, mirror the C drive over, then swap the dives around on the ATA cable. Then you can clear the (previous) C partition on the old drive. Drive letters will automatically re-assign.
Most new retail drives come with mirroring software. If it's an old, or OEM drive you can probably find free mirroring software at downloads.com
Of course, make sure the the computer is powered off with the power cord disconnected and touch the chassis to discharge any static electricity before physically installing or swapping drives.