You are probably comparing different versions.
OEM Full Version is like a disposable camera. You are legally permitted to install it on one single computer, EVER. You can reformat and reinstall it on that same single computer, but never on a different one. Not even if you uninstall it from the first computer. Disposable. It's licensed to that computer's hardware. From what I've read, the motherboard is the tiebreaker if you have to decide whether your computer has drifted far enough from its original hardware that you need to re-license.
Retail-boxed Full Version is not licensed to the first computer you install it on, it's licensed to
you. You can uninstall it from Computer A and install it on Computer B, you're legally permitted to do so.
Retail-boxed Upgrade Version upgrades a qualifying previous Microsoft OS (Win98 for example). For your upgrade to be legit, you must have a license for the previous version, not just the CD-ROM itself.
Typically the OEM full version of XP Home Edition is under $90, the retail full version is about double that, and the upgrade version is inbetween. Here is more info on the OEM do's & don'ts:
http://www.microsoft.com/piracy/YourPC_do.mspx
Another common misconception is that you can transfer a preinstalled or OEM copy of Windows from an "old" machine to a new machine. An OEM software license may not be transferred from and old machine to a new machine?even if that machine is no longer in use. The OEM license is tied to the machine on which it was originally installed and can't be transferred to other machines.