When you have a dual boot setup, you have an option of choosing what operating system you want to log in to. The benefit is that if one of your operating systems screws up, you can just boot into the other one. Kinda like having to computers in one...
To do it, all you have to do is make two seperate partitions on your harddrive. One for Win98 and one for WinXP. Then you should install Win98 FIRST(always install the older OS first), on to the Win98 partition. After you are done with Win98, install WinXP on to your WinXP partition.
After you are done installing XP, everytime your computer boots up, there should a menu that XP makes automatically because it detects that you have Win98 already installed. It's simple really.
I'd recommend Win 2000 over Win 98 for the second OS though.