You might find that the Windows 7 partitioner is unable to see any space on the hard drive, as the OP probably used ext2/3/4.
Go to http://gparted.sourceforge.net
and download a gparted iso.
Burn the gparted iso to a CD or a USB drive (documented all over the internet)
Boot from the CD or USB drive and follow the on-screen instructions.
Once the gparted desktop shows up, it should auto-load gparted within a few seconds. This will automatically scan for your hard drive, and show its partition table.
Right click on each item in the list and select delete.
Once this has finished, turn off the computer, removing the USB drive or CD, and insert the Windows 7 installation CD.
Follow the on screen instructions.