I chased false limit switch triggers for weeks with no success.  My solution was to introduce a 12V DC relay - the six limit switches were wired in series and were in the 12V supply circuit to the relay coil.  The normally closed circuit in the relay was wired to the limit input of my controller.

Works well.  Not the best solution, but for the life of me I couldn't work out what was causing my limit switches to trip - and I tried to be very diagnostic.

I chased false limit switch triggers for weeks with no success.  My solution was to introduce a 12V DC relay - the six limit switches were wired in series and were in the 12V supply circuit to the relay coil.  The normally closed circuit in the relay was wired to the limit input of my controller.

Works well.  Not the best solution, but for the life of me I couldn't work out what was causing my limit switches to trip - and I tried to be very diagnostic.

That's a good idea, because I know that the cause for my limits to trip would only be a few miliseconds, but not long enough to activete the relay. I guess you could say that the relay is being used as a time delay. It's been a busy week, and hopefully will get time to mess with my mill later this week.

You say its not the best solution, I disagree, it is
Its the way Industry does it, well normally 24v but same principal ie much greater difference between a low and a high signal so lot less chance of false triggering from noise.
Hood

Another option for the noise in the supply line is a noise filter.
Cheap and easy to implement in any electric/electronic machine and easy to find in any good electronics supplier (RS/Farnell/Conrad etc).
I always have a couple of them in my parts bin.
It filters the noise in the power line into the machine and/or computer, most of the good PC power supplies have one.
Airborne RF/HF noise is a complete different story

Ric

The problem is with the welding. Think of it as a radio transmitter. Remember when CB radios would bleed over into the tv and stero? I wonder if putting a grounded wire screen would trap the interferance?

Try a metal case for the computer or at least a metal box to help shield the eveything. Move the TIG away from the machine.

Hal

Faraday cage any one ? 
http://science.howstuffworks.com/faraday-cage.htm

Ric

Well I still have not solve my problem, but here's where I'm at now.
It's not the limit switches causing the problem. When I activate my welder, the breakout board is what is messing up !! I've tried putting a capacitor across the 110v input to the B/O board but that did not help. I'm totally lost, and can't narrow down if the interference is coming from the wires going into the B/O board, or if the interference is air borne. 

The noise is being picked up by the parallel cable going from the BOB to the PC...or possibly by the PC itself.  We deal with HF noise all the time working with plasma machines.  Industrial plasma power supplies use an HF arc start.  No different than your HF TIG.  Did you ground the PC chassis to the star ground?  The other thing you can try is soldering a ground wire to the shield on the DB25 cable and grounding that to the star ground.  You would only ground one end of the DB25, which is usually the BOB end. 

This morning I drove an 8ft ground rod right through the floor and into solid ground beside my mill and attached a ground wire to it. I brought the pedal from my TIG and placed ot the table of the mill so I could just activate it with my hand ( my welder is in a different room, but the pedal reaches). It tried grounding different areas in the control panel as well as the case of the computer and the cable. Still no luck !! I even ran an extension cord to power the computer tower so that the 120v feed for it came from a different area. It seems like the component that is causing problems is my breakout board ( Campbell all in one ) but there's no ground provisions on it. Like I said before, the Breakout board goes nuts when I activate my TIG and when it does it activates my VFD. I'm lost as to what else to try !!
Have a Merry Christmas !!

Is the BOB in a metal box? A simple one could be enough ti ground the board.