For the most part, I believe that you can't interchange Desktop and Laptop CPUs. I could be wrong though. In the case that I am, as long as they have the same socket, and the motherboard supports an i7, then you should be fine.
You're not being daft, but as enemy1g states, unless its the same socket (very unlikely) and the power management is the same (possible, but also unlikely), you won't be able to exchange the two.
Thanks for the replys - I was thinking that mite be the case because I have this big ass heat sink & fan keeping it all cool! That's obviously not in the laptop
On the ASUS website:
http://www.asus.com/Notebooks_Ultrabooks/ASUS_VivoBook_S500CA/
It does say your Laptop can receive up to an i7-3517U...
So you could just try to find that processor and swap it out