The source of EMI from PCs is the PC itself. With speeds faster than 1GHz, and bus speeds ranging from 100-200MHz and cables and copper interconnects (read: antennas) all through the PC with speeds of MHz to GHz, there is all sorts of broadband radiation just waiting to escape. There's very little you can do about it.
PC monitors can also give out tons of EMI. This is easy to check- just turn off the monitor and see if it stops. I had a huge argument with one of my coworkers once due to high-audio frequency EMI interference. The problem was that I was the youngest in the office by 10 years and nobody else could hear it! Was this ultra-high-pitched squeal that was driving me nuts, and he was insistive I was making it up and refused to do anything about it, until a team of fire inspectors came through and the youngest of the group, a 20-something new hire, commented on it, too.
In this case, all we had to do was change the refresh rate of the monitor from 60Hz to 70Hz and it pushed the EMI out of my audible range where it was no longer annoying. I swear it still gave me headaches, though.
