tools such as
www.sysresccd.org
is free, can be booted from anywere, CD, HD, USB, RAM
contains DBAN
but since you want to target specific space on the disk id suggest just...
http://en.wikipedia.org/wiki//dev/zero
it default boots to a linux shell but you can start the GUI by typing 'wizard'... use Gparted to create a partition on the half of the drive you want wiped then /dev/zero it from console aka Terminal: first example in the wiki link
lota people here also suggest
http://partedmagic.com
I havnt used it so didnt even realize what it was for a while because its name sounds like a partitioner lol
but its apparently very similar to sysrcd, except more GUI so userfriendly. i havnt used it though, and dont know if it has the aplications such as DBAN.