Amen-Moses said:
I paid 139 UKP for the family pack, that's 5 licences, for Tiger. As I have 4 Macs that makes sense.
A single user copy of XP-Home is currently selling for 159 UKP (and that is a "web exclusive" price, in-store it's 180!) from PC World.
OEM copies are useless because if you change the mobo it will no longer work, i.e it is a non-transferable OS which stays registered to the first person/machine combo you tell MS about.
Even then the OEM price for XP-Pro is greater than the single user price for Tiger.
Amen-Moses
OEM does not become "useless" if you change motherboards. The system builder XP Pro OEM that you can get, as with the link I gave above, has to be purchased with hardware (a $5 mouse will do), and is cheaper than OSX retail and works just fine with any x86 hardware. XP Home OEM can be had for $69.
You might have to reactivate if you have substantial hardware changes but it doesn't quit working. Reactivation requires nothing more than a 5 minute phone call where you assure the MS representative that you don't have a copy of the OS currently installed on other hardware, but they don't check in any other way. I've done this more than once when upgrading my own home computer or changing hardware at work.
An exception to this are versions included with premade computers (such as a Dell) that can only be loaded on hardware from that company, but will install on other computers from the same company (which you can't complain to much about being an Apple fan, right?)
Edit:
A quick google search found this:
http://www.planetmicro.co.uk/product_info.asp?stockcode=M000166
XP pro for 89.95 GBP. Still not as cheap as I've seen in the US, but on a par with what you'll pay for OS X. (And yes, I do realize that XP is a couple of years old now)