One of my motor mount brackets (the bracket bolted to the block) has a couple of cracks in it... I'd like to have it welded but I'm worried of possibly frying the computer or electronic equiptment... Any chance of hurting anything even if I disconnect the battery or unplug the PCM? Or am I being overly paranoid?

 

Def unplug your grounds like you said (i think you said that)
Why not buy another mount?

 

make sure you put the ground for the welder near the spot you are welding on and then you shouldn't have any problems as the current will only travel from the electrode to the ground.