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#11
Chances are you have one HD with one partition anyway, which will show up when you select Custom install. Then click on Advanced drive tools so you can format that partition before installing.
Check now to make sure by clicking Start orb> then right click Computer> select Manage > then Disk Management. Look on the drive map and see what partitions are listed.
This is similar to the map you will see when you select Custom install from the booted Win7 installer.
Do you see where you will install Win7?