For those of who who find this on a desperate google trawl like myself.
Inside the windows 7 installer, you can launch command prompt with CMD F10, then follow the previous post's advise with regards to diskpart...
Once you've entered the command prompt, type diskpart to enter the
Type select disk # replacing "#" with the drive number of the one you
wish to format. To see a list of disks, type list disk. Type clean.
This deletes all volumes from the drive. Type 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.