I have 3 systems on my computer, but they do no duels. Would be fun though to watch them do so. Suse Linux vs. Windows XP Pro...
I surf the Internet with Win98 (or Linux, with which you don't have any malware problems) and work with Win XP. Maybe that's complicated for some, but it works fine with me and I don't have to bother about the newest security leaks in Windows XP.
Normaly you install the newer (WinXP) after the older (Windows 2000) OS. You need a second partition where you can install the additional OS. Installing both in 1 partition is not recommended.
Dual boot is having two or more operating systems together on one system.
Dual boot system has an advantage that if one operating system doesnt start because of a problem then you can start the other operating system and repair the other. But dual boot is recommended only when you have large disk space. I have three operating systems 2000, XP and 2003 server.
What are you planning for ?? a clean install of win2000 or an upgrade ??? you cant upgrade to a lower version of windows ... so XP cant be downgraded to 2000.
For clean install you can install two windows on one partition but its not recommended.