You need to boot into a different system in order to reformat the drive. You need to boot into ANYTHING else using a CD (your Win7 install media) or something like a Linux live CD (like Ubuntu). Either one should give you drive tools like FDisk to wipe and reassign partitions. Obviously only do this if you do not want any of the original data from the drive. You did make a backup, right?
For further help, please post in the proper forum section.