You bought a machine and have a license to run XP on it. that does not give you the right to install another Operating System for free.
If you have an unused copy of Win2k, then you can install it, but if that copy is installed on another machine, you are violating the EULA.
You can use the XP on another machine, too, as long as it is removed from the original machine.
Basically, your license with Microsoft means you can install an OS on any machine, as long as it is installed on only one machine. Win2k and earlier also allowed you to install the OS on a laptop that would be used by one person. So, I have Win2k installed on my office computer, and the laptop I have been issued has the same OS on it, from that CD, and since the laptop is used solely by me, the primary user of the original install on my owrkstation, it falls within the EULA, saving us the cost of buying 2 copies of Win2k.
Basically, have a good read of the EULA!