As per the Microsoft System Builder Website:
When would my customers want to use downgrade rights and how would they benefit?
Your customer is purchasing a new computer system and it needs to run the same operating system as their existing 10 systems—Windows® 2000 Professional. You can preinstall Windows XP Professional on the new computer system, then offer your customer the added service of installing Windows 2000 Professional on your customers' behalf. The downgrade media must be supplied to you from the end user and must come from a legally licensed version of Microsoft retail, OEM/System Builder, or volume license channels. Your customer can run Windows 2000 Professional until they are ready to upgrade the system to Windows XP Professional—and at that time they would already be licensed for the Windows XP they purchased with this new machine.
And a few other points:
In the event the End User opts to downgrade, neither Microsoft nor the PC Manufacturer is obligated to provide customer product support for Customer Systems on which such previous versions are installed.
The software may not be installed on any other computer system than the one which was downgraded under this right.
If downgrading Windows XP Professional, the End User cannot use the downgrade operating system and Windows XP Professional at the same time.
Basically, you can wipe XP Pro from your new HP and put Win2k Pro on it. Now of course your not suppossed to use that XP Pro license on another computer, but that doesnt stop people from using that XP Pro cd-key.