PC's emit a good amount of both EMI and RFI. Between the power supply, CPU, and bus I don't know how you could shield the soundcard and most importantly the DAC chips on it. Special care would have to be taken not to short anything out. You might try using ERS cloth over the whole soundcard or just some bluetak or two sided sticky jell on the DAC chips and putting a small piece of copper shielding on top of that. Probably should ground the shielding to the PC chassis.
But the best and easiest thing to do is just use an external DAC.