Boot off your Vista disc and
go to the Recovery options. Once inside there access Command Prompt (It
is a choice in the Menu) type Diskpart.exe
and hit enter. Then type Help you will see many options.
What you want to do is type 'List Disk' select the Disk you wish to
convert and so forth. Basically the way to convert from GPT to MBR is in
there. You can also clean all data off the disk from inside Diskpart
and start over with MBR.
Commands to Convert Disks
convert mbr [noerr]
Use the convert mbr command to set the partitioning style of the current
disk to MBR. The disk may be a basic disk or a dynamic disk but the
disk must not contain any valid data partitions or volumes.
convert gpt [noerr]
Use the convert gpt command to set the partitioning style of the current
disk to GPT. The disk may be a basic or a dynamic disk but it must not
contain any valid data partitions or volumes. This command is valid only
on Itanium-based computers; it may be unsuccessful on x-86-based
convert dynamic [noerr]
Use the convert dynamic command to change a basic disk into a dynamic
disk. The disk may contain valid data partitions.
convert basic [noerr]
Use the convert basic command to change an empty dynamic disk to basic.