For Vista, I am running GNU which emulates EFI. But no go, Vista wants an MBR to install on so does Windows 7 RC. I am hoping that Windows 7 would remove this limitation. Also, Windows 7 creates two partitions, one boot partition, the other system file partition, that limits the user to a maximum of 2 additional partitions under the MBR partition scheme. This is very limiting on 2GB hard drives where most users will want at least 4 partitions, not one giant one. GPT gives you up to 128 partition capability.
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"16. Can Windows Vista and Windows Server 2008 read, write, and boot from GPT disks?
Yes, all versions can use GPT partitioned disks for data. Booting is only supported for EFI-based systems. "