DOC works this way: when booting up, it looks for a bootable C: hard drive IN THE BIOS configuration, and will install itself as the boot drive only if there aren't any.
Thus, in principle, it is not possible (except maybe patching the DOC firmware) to boot from DOC when having an HD properly configured as C: drive.
However, as modern operating systems handle the drives on their own instead of using the BIOS, Windows or Linux will indeed recognize an use a drive that is NOT configured in the BIOS.
I am not sure if having a single drive in a system configured as slave is good, but just in case I would do the following: connect your HD as MASTER, no slave present. Then go to the BIOS and set all HDs to "None". This way, the system will boot from DOC, but the OS will see the drive with no problems.
This information is 100% tested for Linux. I believe it is true too for Windows according to what the original poster said.