it's a bit complex and it will be unique to your system, i can give you the general gist of it.
first, make a back up of your good system just in case.
you can use the recovery consoles 'bootcfg /rebuild' command to detect each of the operating systems in your computer, (instructions in the following link) then log out of the recovery console and reboot, you should be prompted with an option of which OS to boot from, choose the second, and that should be the second OS. once you've confirmed it working on the SATA drive, you can delete everything on the PATA drive except for the things in the root directory.
http://support.microsoft.com/kb/291980
however, I'm not sure if your up to this, its complex and you can hose your operating system by mistake (thats why i said make a back up). and you must keep in mind if anything happens to that PATA drive, your computer can't boot. and if you accidentally delete the important files when your system is going, you computer will be fine until you reboot, then it will be screwed.