Converting on a clean drive can be done with GParted, Partition Wizard by Minitools, or diskpart
With Gparted the device you wish to be GPT is selected in the upper right dropdown box.
Next would be to create a new partition table as GPT that deletes all data on the drive if present.
In the menu bar select Device > Create New Partition Table. Select Advanced and then GPT in the dropdown box.
Partition Wizard can do this on a non system drive without data loss but the data should be backed up first as a precaution. The new version 10 is supposed to be able to convert a MBR system disk to GPT.
Note: A drive can be converted to GPT using diskpart. Using this method, like Gparted will remove all data on the disk.
Boot the Windows install disk. At the screen that asks for language press Shift + F10 to open a command prompt.
Type the following commands at the command prompt
diskpart Opens a diskpart prompt window
select disk X Where X is the disk Windows will be installed to, usually 0
clean Deletes all existing partitions on the disk
convert GPT Creates a GPT disk partition table
Hope this helps.