The only way I know how involves using another OS: BeOS or Linux. Basically you need to partition your hard drive. This can be done in many ways. There's serveral commercial partitioning programs out there. You should get one if you buy a new hard drive retail. There's also different ones on the different distributions of Linux. You'll need at least one partition for each OS. Then you'll need to install each OS on it's respective partition.
Okay, here's the hard part. You then need a boot-loader to load whichever OS you want at boot time. The Windows boot loader, obviously, ony allows you to load one version of Windows. Both the BeOS boot loader and the Linux boot loader, LILO, allow you to boot Window partitions as well. All versions of windows need to be installed first. Then install whatever other OS you want and the ability to setup the bootloader to boot multiple OS's should be in the install.
I don't know of any commercial boot loaders or about the boot loaders of Windows NT or 2k. I am also almost positive that there is an easier way to do this, unfortunately I don't know what it is. I hope someone else can help because I'm wanting to get a copy of Windows 95 to play around with and dual boot with Windows 98 SE.
I know of two boot loaders. I use System Commander, but there's some other one that I've seen on the store shelf too.
Dual booting Win98/Me is like deciding whether to beat your self with a steel ball bat, or an oak wood billy-club. At the very least, I would suggest you try a completely different OS. There's other fish in the water, and some of them are much more nutritious.
Boot Star is the other boot loader. Never used it though.
Linux is rather different (read:whole-new-world). It will take quite a bit of learning just to be able to do everyday tasks, not to mention tweak your display/modem/joystick/speakers. Programs may (or may not =) crash, but the OS does not (should not) become unstable enough to have to reboot. Unless you're running for 30+ days during major solar activity.....but, that's a whole different thread, so I'll leave it alone.
I've never used BeOS, so I'm clueless on that one. I've heard a few good words mentioned.