If you have a Windows 7 Installation disk, you can enter Repair mode and use
diskpart to accomplish this.
From Windows 7 DVD, click Repair your computer when you see this screen:
Then select Command Prompt in the Repair window:
Once you've entered the command prompt, type
diskpart to enter the utility.
select disk # replacing "#" with the drive number of the one you wish to format. To see a list of disks, type
clean. This deletes all volumes from the drive.
convert mbr to convert the disk to mbr.
Note: You can repeat this process and in step 3 replace with the command
convert gpt to go back to GPT.
Sources: Change a GUID Partition Table Disk into a Master Boot Record Disk, Change a Master Boot Record Disk into a GUID Partition Table Disk