You can also boot into the Windows 7 DVD, select "Repair My Computer" on the second screen, then from the Recovery Tools list open a Command Line and type:
SELECT DISK # (for Windows 7 target HD)
CLEAN ALL (the "all" will zero the HD which is the deepest clean possible)
CREATE PARTITION PRIMARY
SELECT PARTITION 1
Now boot back into Windows 7 DVD, plug ethernet, select "Connect to Internet during Install" to get latest drivers/updates, select Custom, Advanced drive tools, apply quick Format again, then install.