I work for an ISP provider and my PC had the exact same sounding problem. Wife connected at 18mbps and I got 3.5. I used some equipment from work and proved an 18 Mbps at my PC as well but my speedtesting was giving an incorrect result. I updated my drivers for my NIC, reverted all of my 'connection boosted' settings back to original and restored my registry from a month prior...voila! 18.5mbps. Not saying this is the problem but if you have used any software to boost your connection or you have not updated your drivers recently it could be causing the problem.
First off I would be curious to see what your connection speed is without the router, swap ethernet cables with wife (hers is obviously in good shape). Also, what are you using to speedtest? How old is your PC and how old is her laptop?