You can, but it has to be on the same computer that it came with and you can only run one copy of it at a time.
MS is known for it's strange policies...
If it isn't OEM you can have it on as many computers as you want but it can only *legally* be running one copy at a time.
The only way you can *technically* run an OEM on a "different" computer would be to use virtualization software in another OS such as Linux or Mac with the OEM copy of XP running in it.