I got a question today: Can you convert an existing MBR disk to GPT?
This is about the partitioning style of a disk, with MBR being the older one (limited to 2TB) and GPT being the newer one (going up to 18EB). Keep in mind that the GPT partitioning style is not recognized by all previous versions of Windows.
The answer to the question is yes, you can convert an MBR disk to GPT, but only if your disk is unpartitioned. If there are no partitions on the disk, the option to "Convert to GPT disk" will be available in disk management.
You also have that choice, obviously, when you are partioning an unintialized disk, as shown in the picture below:
You can see this information and a lot more at http://www.microsoft.com/whdc/device/storage/GPT_FAQ.mspx.
You should also look at http://support.microsoft.com/kb/302873.