already in the machine. (wonder what sort of machine that would be?)

Now fire it up and hold down the Option key until you see Startup Disk choices on the screen.
Choose the DVD and go from there.

This previous software disc, is it a retail copy of Mac OS X ?

If all that fails, try booting the machine with the "R" key held down.

That should launch the recovery partition on the hard drive.

Good luck

P