Problem with new rig

Hello, first time poster here. I recently built my first rig and everything went swimmingly upon first boot up or so I thought. Like an idiot I bought and used the Seagate Hybrid Drive ST4000DX001 4TB MLC/8GB 64MB Cache SATA 6.0Gb/s NCQ 3.5" Desktop SSHD for the primary drive in the computer totally unaware of the limits on MBR space and partitions. I installed windows 7 home premium 64 bit on it only to discover that half of the space is now unusable and "unallocated" and I have no clue how to get the other half of my money's worth.

Any help that anyone can provide will be greatly appreciated.

Reinstall, wiping out the old install, this time selecting GPT....

I have no idea how I would do that. Sadly I am entirely computer illiterate so I would need more detailed instructions on how I would do this.