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.
Best Answer
This is done the same way with external or internal drives.
Start your Disk Utility (Applications -> Utilities -> Disk Utility). In the left pane, select the disk for which you wish to change the partition table (select the disk, not one of its partitions, disks are at the "top levels" in the tree view in the left pane).
In the right pane, select the "Partition" tab. You must repartition your drive in order to change to GUID, so in the "Partition Layout" dropbox select "1 partition" (or however many you want).
Click on the "Options..." button right below the list view showing your partitions and select "GUID partition table". Adjust your partitions as needed and hit "Apply".