For 2000 Pro, you can install it once; for XP Pro, you can install it once. I don't think Steven exactly understood what Vegaslon said but he was saying you can install an Operating System only once - there are no laws about how many Operating Systems you can install on one PC. Vegaslon was talking about the individual Operating Systems when he was talking about installation. Linvx was not even talking about Dual Booting; so why Vegaslon would be talking about it, when he never said anything about it, is confusing.
Microsoft will not approve you to install one copy of Windows 2000 Pro or XP Pro onto two machines; its not in the licence of any. You'd need a Corportate Licence named to you and the versions you have. That's the Legal Standpoint of course. Technically you can install 2000 Pro as many times you want and nobody will notice. XP Pro will not work if you install it more than once and if you do get around it - you won't be getting any Support on either machine because of Genuine Advantage. Updates are important on a Windows Machine.
A Knight is sworn to Honour. His heart knows only Virtue. His blade defends the helpless. His might upholds the Weak. His word speaks only truth. His wrath undoes the Wicked.