Is the pc looped back through the mixer?
Does your system have noise on the speakers as well as the rec o/p?
If so, then you need to check all your grounding and signal cables...
If not then ensure the RCA lead to the pc doesn't run near power, Flouresent tubes, Light controllers etc... then also check the PC PSU is grounded correctly. - Plug in another line device and test it... some cheaper sound cards suffer with this because of poor grounding an signal isolation.. Also ensure that the O/p from your mixer does NOT exceed 0.77v and that it is plugged into your line in, not mic in...
Hope this Helps !!!
Mad