You're going to have to explain what you mean by "interference." But I'll explain based on my first guess.
Assuming that it just makes your TV screen look fuzzy, I've had similar problems with appliances like a vacuum cleaner. DC motors create all kinds of noise on the line they are plugged into, and some how (I'm not sure exactly) this feeds back into the TV. Since it actually makes it to the monitor of the TV it has to be fairly high frequency noise (as I think channels start at like 20 MHZ).
So my first guess would be to put a coupling capacitor on the line. Which *should* filter out any of the high frequency nonsense, the problem is you're working with dangerous voltage and current levels so you must be very careful.
But what I would do first (if you have a working knowledge of some advanced techniques, I'll tell you what I'd do). I'd scope both the + terminal and the -terminal on the drill and do an FFT of the signal so I could find out what type of frequencies I was dealing with, and then I would design a simple low pass filter to get rid of all of the extraneous noise.
Good luck, and be careful.
Oh
P.S. You might try plugging your drill into another socket that isn't on the same circuit as the TV, that might even be a simplier fix.