Its not all that big a deal, but in order to configure a machine for dual or multi-OS boot capability, each operating system hasta be on a seperate partition on your main, or root, drive. The number of partitions on a drive generally are set up at the time of the original format-and-install with the original operating system. Normally, a consumer computer with the factory-installed operating system will have only one partition. Now, that's not necessarilly a dead end; some commercial utilities claim to be able to create new partitions without disturbing what's already on the drive, but it don't work all the time, and sometimes seems to work, only to turn around and bite you badly at some particularly inconvenient time and manner.
I would suggest you copy all of your important personal data to removeable media (CD, DVD, ZipDisk, whatever), gather fresh downloads of all of your necessary drivers - machine and peripherals - for Win2K (or XP if you're going that way), make sure you have installation media and product keys for any software you've purchased, then do a full format-and-install with whatever OS you want to migrate to. You might also want to get yourself a free web-based email account, such as HotMail, YahooMail, GMail, or the like, forward any emails you really wanna keep and transfer your addressbook data to the webmail client, too; you coiuld also export your folders and settings to another folder and burn that to removeable media, which is not all that hard to do in most cases. Lotsa work, yeah, but it works - every time. When you do the format, you can set up and size partitions as you find appropriate to your needs and intentions. Once you've got the new OS on, and partitions set on the disk, you can load another OS, such as your old one, onto a different partition. As mentioned, any program you wish to use cross-platform will have to be installed in both operating systems, but files and folders should be pretty much accessible and manipulable between the 2 (or more) operating systems.
You also could try an "In Place Upgrade", installing the later OS over the earlier one (and its gotta be done that way - the newer OS hasta go over an older one, the other way around won't work), and more or less transfer all your files and settings while replacing the older OS with the newer one, but that can have some entrtainment potential, too.
Gotta say the hard way here - format-and-full, clean-install - "Starting from scratch" - is the best, most trouble free, likeliest-of-satisfaction method.
See Microsoft:
Multibooting with Windows XP (pretty much same-o same-o for Win2K)