I'd suggest looking into virtual machine software, and using those. Otherwise, you'd pretty much have to reinstall Windows AFTER installing DOS, because DOS will wipe out the Windows boot loader without even bothering to ask.
I have a new desktop computer with Windows 7. How might I create a DOS partition in the unused portion of the new hard drive. I would like to be able to use QuickBasic v4.5, and able to share the files I create in the DOS volume with the primary NTFS 64bit OS volume.