Reinstall OEM Windows 7

[clv101]

I have two PCs both around 4 yes old onto which I installed OEM Windows 7.
I'd now like to reinstall windows but I don't know which serial number us which... Is there a way to find out?

 

[KIA]

http://www.magicaljellybean.com/keyfinder/

 

[demonix]

If you haven't done a reinstall before, then what has been suggested isn't going to help if it's a manufacturer built system as they use a generic image to install the OS onto the computer which uses a key that can't be used for activations on reinstalls.

You should have a sticker with the key on if it is a manufacturer built system.

 

[clv101]

I built the machines myself - I have the Windows 7 boxes/stickers here, just two of them and don't know which key goes with which machine.

KIA's tool cleared it up, thanks.