How can i delete a windows partition to have the single partition i used to have?

Im trying to delete a windows partition that i created a while back in order to make a single Mac partition, but I'm having difficulties. I've successfully done it before running Boot Camp Assistant but now I'm trying to delete the partition utilizing Boot Camp Assistant but as soon as i hit continue i get the following message:

"Back up the disk and use Disk Utility to format it as a single Mac OS Extended (Journaled) volume. Restore your information to the disk and try using Boot Camp Assistant again.

I dont need the windows partition so i want to completely get rid of it. I dont have a seperate harddrive to back up all my stuff and i want to start fresh with a new copy of Mac OSX leopard (with a single partition), of course after i find a way to delete this windows partition. Someone help me please!!

If you want to start fresh and are not concerned about data lose on the Mac partition then boot to the Leopard DVD and run Disk Utility off the DVD to repartition the drive to a single partition, then continue to install OS X.