How can I be sure I'm not being spied on?

Hi, I'm buying a number of used computers from a competitor that I don't totally trust. How can I be sure they haven't planted some kind of spying device in the computer. Someone recommeded I replace the NIC card is that a good idea? Any good software programs I can run to scan the computers?

Thanks for your help.

I Say you just reformat the pc, wipes everything off.

Replacing the NIC card has nothing to do with it That just connects the pc to the internet. I never heard of spying NIC cards LOL.

Best Bet is to format them. They software on it is prob not Legit anyways.