If you're formatting your hard drive for a clean install of Windows, before you do, make sure you de-authorise the machine first by going to Crysis 2\bin32\ and running Activation.exe. When the window pops up, it'll tell you your CD key, how many activations you have left and in the bottom left corner is a button saying "De-Authorize This Machine", hit that and it'll do exactly as it says on the tin.
You can then reinstall windows without fear of having lost an activation, as I am not sure what will happen if you reinstall windows before deactivating, I guess one of your activations just disappears forever?
Although, that said, I have the game activated on both my home PC and my laptop, and it still says I have 5 activations...
I hope this helps someone!
But, what if I can't play online when i de-authorize? I will wait for a Crytek answer

Thanks