For the most up-to-date licensing information, refer to the microsoft web site.
Retail
2000 Pro, XP Home, XP Pro, Vista Home Basic, Vista Home Premium, Vista Business, Vista Ultimate.
you may transfer the software to another computer (nothing mentioned as to how many times, though a situation like this calls for a rebuild and if enough HW changed, it may trigger a need for an activation call to microsoft with an explanation as to what you were doing). you may transfer the software one-time to another person. You are also supposed to transfer the Proof of License label.
you could not use an OEM XP installation CD to downgrade a copy of Vista, because the OEM license was restricted to one machine (both OEMs, both legally purchased). You must purchase a legal copy of an XP Pro Retail cd.
Upgrade
2000 Pro, XP Home, XP Pro, Vista Home Basic, Vista Home Premium, Vista Business, Vista Ultimate.
license for XP Home upgrade says license limited to 1 CPU. I can't remember, but I thought it said 1 workstation too. maybe it said 1 CPU, which makes sense.
OEM
2000 Pro, XP Home, XP Pro, Vista Home Basic, Vista Home Premium, Vista Business, Vista Ultimate.
the package basically says "to be distributed only with a new computer". and from what I can tell it is meant to stay with that computer. That's why you're not paying the big bucks for the retail version of Windows.
you could not use an OEM XP installation CD to downgrade a copy of Vista, because the OEM license was restricted to one machine (both OEMs, both legally purchased). You must purchase a legal copy of an XP Pro Retail cd.
You can buy the OEM with either a new system, or buy with a piece of hardware. The OEM version with the computer it is installed on.