Shoutbox

As TReKiE said, typical PC-generated interference consists of a wide range of Nth harmonics and intermodulation products from the front-side, memory and PCI busses.  Even if you were able to change these, you would at most shift some of the peaks, while not necessarily eliminating the offending interference.

However, on most PCs, there is a BIOS setup option (somewhere in advanced or chipset settings sections) for "spread spectrum".  You may wish to enable it, as it is there specifically for reducing radio frequency interference to other devices.  It slightly modulates various clock frequencies, so that the RFI emissions are distributed more evenly over a wider spectrum, with fewer sharp peaks on any one given frequency.

Beyond that, the only thing you could do would be try to limit the ways for the interference to escape.  The main ways it gets out are direct (solvable by improving the shielding qualities of your case) and fed back through the power lines (better power supply or RF filters for the power line may help there).  Shielding of external cables connecting the components should be looked at, as well.

I suspect you don't want to tackle either of the above, as the solutions that work are usually not simple fixes and involve expense (if you were to replace your case or add power line filters), or at least a lot of work (lining with foil and what not).

P.S. Is this a major OEM computer (e.g. Dell, HP)?  If so, there are regulatory guidelines for the maximum allowable interference levels a computer may emit and you may have grounds for a warranty claim.