Trade-in or trade in?
‘Trade-in’ is a noun. It is something that is accepted as part-payment for something else. That’s why PC World needs to drop the hyphen in their ad: ‘Up to £100 off any computer when you trade-in your old laptop’. In this case, your old laptop would be the ‘trade-in’, so, in effect, the ad reads: ‘Up to £100 off any computer when you laptop your old laptop’.
You will actually need to trade in your laptop to take advantage of this particular offer.