Whether or not you can do this at all will depend partly on on (a) whether or not you have checked baggage and (b) what passport(s) you are travelling on. It also depends where in Cambridge you need to get to for 7 pm.

Trains from Kings Cross to Cambridge run at times .15 and .45. According to nationalrail.co.uk you need to leave Heathrow (on the Piccadilly Line) by 1636 to catch the 1745 (arr 1836), or if arriving at 1906 is OK for you then you have to leave Heathrow on the 1706 Tube - which you should be able to make even with non-EU passports and checked baggage. (I don't think it's going to be any quicker using the HEX)

As for driving... as already mentioned it will be the height of the rush hour, so you'll be in heavy traffic all the way. Arriving at 1600 it's unlikely that you will be hitting the road much before 1700, and I really don't think you can make it to Cambridge in two hours at that time of day... unless it's somewhere that's just off the M11.