This is a hardware problem (maybe is possible to mitigate via software), particularly on Dells. The touchpad doesn't have an stable ground (0 V) level. You can of course try a live distro to confirm.
Workaround: A way I found helps in all cases is to touch with your other hand the ground, some way to touch a metal part of your laptop. One easy way to do this is to plug an ordinary USB to microUSB adapter and touch the metal shield.
Another way is to use a cable or a paper clip (shaped like a 9) around the AC plug.
Sometimes changing the AC power adapter helps.
Of course to avoid all this you can just use an external mouse.