Ferrites are my friends. You can never have enough clamp-on ferrite RFI filters. Unfortunately just about every radio and components stockist charges a ridiculously high price for them, like £2.50 (about $4) each, which makes it expensive to keep a stock of them. However, a few weeks ago I found an eBay seller in China that was selling packs of 6 or 10 clamp-on RFI ferrites at a much more acceptable price, so I sent off for a bunch of them.
This afternoon the K3 was sitting scanning 2m 144.250-144.350 while I was working, and I noticed a 3 S-point increase in the background noise level. My wife had just switched on her laptop, so I thought that might be the culprit, and sure enough we soon found that the noise was being radiated by the power supply cable. Even if the power supply was off and the computer was running from its battery, the noise level jumped up when the power cable was connected. One turn of the cable through a clamp-on ferrite was all it took to fix it. (Most PC power supplies seem to have a moulded-on ferrite at this point, but curiously not this one.)
A few weeks ago I used clamp-on ferrites to cure a problem with my Elecraft K3 transmitting a poor PSK31 signal (high IMD) on 10m. What seemed to be happening was that RF was getting picked up on the shield of my 2m antenna and coupled back to the K3, causing the problem. I dare say the real cause is a poor RF ground, but it's hard to get good grounding at 28MHz when you're about a quarter wave above ground already. Anyway, fitting clamp-on ferrites to the antenna cables and any other long cables that seemed to be contributing to the problem seem to have cured the trouble.
Clamp-on ferrites have also been quite effective at keeping RF out of the computer. The mouse would freeze when 100W is used on some bands, but looping the USB cable through a ferrite soon fixed it. Without having a box of ferrites sitting patiently in the cupboard, I might have wasted a lot of time trying to find another solution.