In windows you can never format your C drive, its your active partition that you booted off of(if you have multiple windows OS's installed each one relabels the partitions so they all booted off of C) You can never format the partition you are booted off of, you need to be running off of a bootable CD or another partition if you want to format your main partition.
Also, what particular reason do you have for wanting to format your main OS partition? If you do it, make sure you have all of the installation material handy, you will need it all again.
hunter is half right / half wrong. OK, mostly right
You can't format the drive you have booted from, but the drive you booted from might not be C:.
Simplest way to format your normal boot drive is to boot from CD or USB.
However, as hunter said, you have to be certain that you do mean to format that drive, because it will mean losing all the data on that drive. Given that a normal layout will put your Documents folder on that drive, as well as Program Files, you really want to be certain.