Get a Linux install CD for a small distribution like Puppy Linux (less than 100 MB). Don't worry, you never need to actually use Linux if you don't want to, but we do want a minimal installation.
Why? Here's the thing: 99% of all Windows installations are done at the factory, and the Windows OS on the computer will never see any competition. There's just not that much demand or need for Microsoft to put a lot of work into its boot loader. Linux, on the other hand, has historically had to play second fiddle. It's much more likely that a Linux system will have to share a computer with another OS, and it's much more likely to be installed second (which means its boot loader is the one left standing). The result is that the Linux folks have put many times more work into their boot loaders than Microsoft, and it's a much better experience.
So, again, my recommendation is to download a very small Linux distribution (preferably one with GRUB), carve out just enough space on your hard drive to hold it (<1/2 GB should do for really small ones), and install it last, so that its bootloader can detect and show your operating systems in the menu.
This will accomplish your objective. You will see a boot menu that allows you to select an operating system, and no matter which operating system you choose your computer will not need to restart.