The real difference, and this is a point that has been missed here, is that an OEM copy of WindowsXP is not supported by Microsoft.
If you have problems, you call the OEM (Sager, HP, Dell, etc....). Microsoft will not help you out...therefore, since they are not supporting the product, the price is lower. To a degree, they do support the product (updates, for example), but the more mundane aspects are left to the OEM.
For example, I had a retail version of WIndows98 and the CD was scratched. I called Microsoft and they sent me a new CD for free (I paid shipping). I have an OEM version of XP. If I scratch this, Microsoft will NOT replace it.
On the other question--can you install an OEM version on another machine. It depends. Legally, as pointed out, you are entitled to install and version of Windows on one machine at a time. If you build your machine every 6 months, you can install the OEM version over and over again each time provided the previous machine is decommissioned. At times, you need to call MS so they will allow activation again, but I have never had a problem with this once I explained my circumstances.
PCTorque/Sager sells laptops sans OS because they know there are people like us out there who routinely migrate up, discarding older machines, so why sell us something we don't need. Indeed, it is one reason why I bought a Sager.
Practically, it is not always possible. I have installed DELL-badged OEM disks on other machines no problem in the past; I have been told that this may not be currently true, as Dell OEM disks are keyed to the hardware. This past week, I ran into an HP that would not work on a the same freaking machine after a new harddrive install because the drive needs to be "tatooes" with an HP code. (I simply imaged the original disk to the new disk, which transferred the tatoo to get around this).
So, you never know unless you try.