The first thing to do is to look at the top line of the sticker. It'll be something like Windows XP Professional or Home so that tells us the version.

Now we need to look at the line below it to determine what type it is. If there is a sticker on the computer case, its 99% likely to be OEM.

If it says the name of any computer company like Dell, Acer or HP, its OEM.

If it says OEM, its OEM.

If there is no license sticker on the computer case, its possibly a Retail Copy. If there is a brightly-coloured 8.5x11 paper folder with a CD holder and a license key on the folder itself, its a retail copy.
If there isnt a case or sticker on the computer, use either the CD or installed OS to determine what it is.


Here is an example of a license sticker:
Example of Windows XP Professional License Stickers


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