Many hardware devices that put out enough electromagnetic interference, or EMI, to interfere with the reception of TVs and radios have a feature known as spread spectrum clocking (SSC). However, SSC is not well understood by many people. It is usually disabled by default. You have the option of turning it on or off, but it's best to leave it disabled.
Digital systems such as computers synchronize the activity of multiple elements through the use of a clock signal or a pulse. This signal usually exists in a narrow band of frequencies. If it's intense enough at those frequencies, it can violate FCC rules about how much EMI a device rated for certain use can put out.
SSC tries to get around this problem by shaping the emissions -- i.e., varying the frequencies used -- so that instead of the clock using one narrow frequency range, it's spread out over a number of different ones at a much lower intensity.
Many PC devices have SSC functions; the most common are motherboards, hard drives and disk controllers. Turning on SSC may lower the amount of EMI put out by that device at any one given frequency, but there are side effects. First of all, devices that are highly dependent on clocks, such as SCSI devices, may not perform as well. Second, any systems running at higher-than-rated clock speeds will almost certainly crash or lock up if you enable SSC. For these reasons, it's best to leave SSC disabled unless you are in a situation where EMI is a real issue.
Another wrinkle with SSC involves FCC testing and certification. When the FCC tests PCs under Part 15 rules, SSC is enabled if it's available. This means that when SSC is off, the device in question may no longer be within FCC parameters. Again, this is usually only an issue if you're experiencing serious problems with EMI.