Ed Bott has aa article on ZDNet comparing the licencing terms of Windows XP and the forthcoming retail version of Windows Vista (Microsoft’s upcoming successor for XP).
It turns out that whereas with Windows XP you could re-install the OS on as many different machines as you wished (as long as it was deleted off previous machines – i.e. transferring the OS from one computer to another), with Windows Vista that functionality will only be allowed one time and will likely be enforced using the ironically named Windows Genuine Advantage (WGA). Further, if you fail to validate your OS with WGA, Microsoft cripples your system, no longer allowing you to access most of the computer’s functionality.
Because you never know when you might need to transfer your licence to another machine my advice would be don’t buy Windows Vista. There are plenty of credible alternatives, not the least of which is the Mac!