How to Delete GPT Partition or EFI from Mac Hard Disks In Vista or XP
Using the traditional GUI of Disk Management doesn't give you the option to delete or reformat, so fire up the command line.
type DISKPART to enter the diskpart utility
at the prompt, enter list disk as below
DISKPART> list disk
take a look at the disk you want to reformat, and select it with the appropriate number, for example:
DISKPART> select disk 1
once selected, you can delete the GPT / EPI by the clean command:
once done, you can then proceed to format the disk the traditional way through disk management and selecting FAT32 or NTFS.
[19/11/09 edited per comments below "clean disk" to "clean"]