OK you are willing to do a clean install. But first save all important data to an external disk!
For example boot from UBUNTU live CD and copy all important files. When done boot from install dvd and do the shift+f10 to enter command prompt (only 1 harddisk attached!!)
list disks (only 1 disk listed? only then continue)
sel disk 0
list part (is this really the right disk?)
Now reboot from dvd again and do a clean install Clean Install Windows 7