Johnnymushio, Thank you for contacting the Dell Community Forums.  After you click Custom (to perform the clean install), you will be prompted to select the partition on which to install Windows 7.  Since you chose the option for a clean (custom) install on the previous screen, you should have a choice of partitions, and should also have options to delete partitions, create new partitions, and format existing partitions (these can be viewed by clicking Drive Options in the lower right-hand corner).  Once you select the partition and click Next, setup will begin copying files to the hard drive, and will eventually prompt for a reboot.  After that, you will be prompted to create a user account, set a password for that account (this is optional), and will then go through the rest of setup (setting time zone, choosing desktop background, etc.).  Hope this helps.
ive read you hit custom install, but then what do you do?
how do i do a clean install