Retail XP has the same clause as OEM XP: One computer, one licence.
The two major differences between OEM XP and Retail XP are that 1) It won't install unless XP is already there or you have a CD with a previous version of Windows, and 2) It's transferable to another computer providing you take it off the old one first.
for clarity...
the only version of XP that requires there to be an OS already on the computer...or for you to have a cd with the previous version....is an upgrade disk..
the difference between oem and retail...is that OEM (original equipment manufacturer) MUST be sold with "qualifying hardware" and CANNOT (by the EULA) be installed on another machine WITHOUT transfering that 'qualifying hardware"...where as retail can be installed on any single computer at any given time (as in only one machine at a time)...regardless of hardware...