As ACtheTroll says, every PC that comes with Windows pre-installed should have a small sticker on it, either on the bottom of a laptop, or on the side of the case on a desktop.
It will look something like this (probably the top one)
http://www.technibble.com/articlecontent/2007/ 11/cod-sticker.jpg
Next to the text "Product Key" will be a sequence of numbers. These are the unique numbers for YOUR version of Windows.
So if you do reinstall Windows from another persons CD DONT put their number in, put the number on the sticker on your PC.
This should ensure you get the right version of Windows (in the right langauge) that you paid for.