Hi lordoftheplat,
You can do it easily. One of my PCs can boot formm Win98, Win2K, WinXP and three Linux distros all on one HD!! (sssh, Mac OS next!)
Of course there's no reason to have 6 OSs but just because they are there and it's fun to show off it to my geek friends.
If you need to use only 98 and XP, you need to create two partitions on the harddrive, install win98 on the first partition. Then install WinXP on the second. When you reboot, the screen will give you a choice to use Win98 or XP, you pick what you want.
There's also a free program called GAG you can DL, you don't really need it but it will give you a nice GUI to pick your perfered OS.
Oh, forgot to mention, you can also use VM (Virtual Machine). You can download Microsoft's VirtualPC 2007 from Microsoft, or download VMWare. Both are free. If you just want to stay with Windows OSs, Virtual PC is easier to install and use, but if you want to use other OSs, VMWare is a better way to go. The advantage of using virtual machine is that you don't need to reboot your computer, you can switch in and out of any OSs you like because they run concurrently. The down side is it will eat up lot more resources and you need to have a CPU at least 1.5G or higher to run them decently.
B.