Before you start, make sure you have a verified working clone of both partitions/systems.
You will ideally want to have software that can clone partitions, such as either the excellent CarbonCopyCloner, or the equally awesome SuperDuper, both of which will allow for your needs (complete clone) in their Shareware mode.
I don't know what tools are suitable for PC, but find one that does the equivalent for Windows installations.
Reboot from your cloned Mac OSX installation, and prepare to repartition your target Hard Drive. Disk Utility can sometimes repartition volumes non-destructively, but for this I would recommend quick erasing the drive and repartitioning to the desired sizes for each OS, with a caveat for the situation below.
Repartition as desired (ensure GUID, journaled, case-insensitive for the Mac Volume), noting you may need to go through various procedures if you are using OSX 10.7 Lion and wish to retain the hidden recovery partition (ie: you may have to reinstall Lion and restore from your backup, then proceed with Bootcamp installation which will again walk you through creating a suitable PC formatted boot volume which you can later clone your Windows installation back to).
Clone back the OSX Installation that you are booted from to the target Mac partition on the internal drive (if you haven't already restored from this in the previous step). Do the equivalent process in Windows for your Windows installation.
Test each by booting into each and making sure everything works as expected.
Remove all external disks (just for safety reasons). If possible also remove all internal disks except the boot disk or "refresh" your backups.
The proposed command (dd) used improperly can be deadly for your data.
Open Terminal.app and get the disk identifier of the disk containing the DVD partition:
diskutil list
Unmount the disk:
#replace diskX by the disk identifier of the "DVD disk" you found previously
diskutil umountDisk /dev/diskX
Overwrite the first sectors of the disk with
sudo dd if=/dev/zero of=/dev/diskX bs=4096 count=8
This will zero the first 8 physical blocks (or 8 4k/64 512b logical blocks) of the partition and should remove any "DVD hints".
Open Disk Utility and try to erase/repartition the disk again.
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