Let's be careful about drives vs. partitions. A drive is an entire physical storage device: a hard disk drive. A drive is something you can hold in your hand.
The storage space on the drive may be divided up into one or more partitions. Partitions are not drives, they are sections of drives. You cannot hold a partition in your hand.
GPT and MBR are drive-level partition table formats that affect the whole hard disk drive. It's how you tell your system what the partitions on the drive are, how big they are, and where they start and end.
The ancient partition table format from the 1980's IBM PC BIOS firmware era is MBR. The modern format from the 2000's Intel [U]EFI firmware era is GPT.
A drive could either have only an MBR table, or it could have only a GPT table, or it could be a hybrid containing both.
One form of hybrid is primarily GPT but has a "Protective MBR" or "PMBR": a fake MBR that tells non-GPT-aware software that the whole disk is allocated to an unknown partition type. This keeps non-GPT-aware software from thinking the disk is unformatted and trying to format it.
Another form of hybrid is a disk that has both a GPT and an MBR, where the formatting/partitioning software has tried to keep them both in sync. So MBR-only software learns about all the same partitions that GPT-only software learns about.
You can make a GPT-only drive into a hybrid, but since this concept is at the whole-disk level and not the individual-partition level, you can't make a single partition be MBR.
Partitions have a different set of formats, such as FAT32, ExFAT, NTFS, HFS+, and others. These are choices that affect individual partitions, not the whole disk drive.
Best Answer
Like Ramhound mentioned the drive /disk MUST be unmounted to assume this is a windows install grab any live installer (live as in can actually use the system -- aka almost any modern linux installer or a Win rescue cd/dvd if oyu have one handy )
Step 1) BACKUP ALL IMPORTANT data ---confirm it was a valid backup (normally I boot it and check that it runs for this purpose)
Step 2) load up the rescue cd/dvd or live installer also very nice is gparted iso (Gparted )
Step 3) using any of the above Follow either the DiskPART instructs from that Tutorial OR the much more graphical and user friendly Gparted iso (also a great live tool for cross platform oh crap moments --keep a copy on usb myself)
3A) USING GPARTED select ' Device' > >
Create Partition Table> > from the drop-down select GPT (GUID) and thenApplyWIN 7 USER SIDE NOTE:
Some laptops / PCs and win 7 versions DO not like installing with boot GPT+EFI this is nearly non existent issue on win 8 or xp (for those that still use it )