Assuming it could be written and not detected. It would pull either part of the cpu or part of the discrete gpu.

neither part does a lot of hashing.  Say you bleed 10 percent of the cpu or gpu.  maybe 50mh a pc so 20 pc's for 1 gh.

2000 pc's for 1th

2,000,000 pc's for 1ph.

not much more can be bleed off without detection.

So is it possible to make a virus that infects 2mill pc's sure.  and if you are never caught you hash at 1ph. my guess is it would be found out quickly. since all the pc's would be dedicating a decent amount of cpu/gpu/watts to the mining.