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Follow these steps exactly. Ask back if you have any questions. If you dont' feel like swapping your Win7 HD into DISK0 then I can give you commands to run instead.
Power down, unplug H and plug its cable into C so that C becomes DISK0. Make sure C is set as first HD to boot in BIOS setup.
Boot the Win7 DVD, select Repair My Computer on second screen, accept any offered repair. If it doesn't start after reboot, boot back into the Repair console, click through to Recovery Tools list to run Startup Repair repeatedly until Win7 starts. Pictures here: Startup Repair
Power down to plug H back into DISK2 cable. Restart Win7, type CMD in Start Search box, rightclick result to Run As Admin, type:
DISKPART
LIS DIS
SEL DIS 2 (confirm from list this is H HD)
CLEAN ALL (takes awhile, wipes HD of all code)
EXIT
If you don't want to wipe the HD, then instead of CLEAN ALL use DELETE PARTITION OVERRIDE
You can now repartition and format DISK2 in Disk Mgmt.