The first describes the probable cause of most problems, and suggests a couple of possible solutions (and one to avoid):
The second discusses some specific laptop models that seem to cause problems due to a grounded power supply:
There are lots of other FAQs as well - the full list can be found by following the link from our main forum page, or by clicking on the other sticky thread next to this one.
N.B. 02/09/2011 I've just added desktop to the title because so many desktop computer users are now suffering from similar problems. Ignore laptop-only advice relating to power supplies (all desktop PCs MUST be mains earthed), but the advice relating to tracing and eradicating ground loops is identical for both desktop and laptop computers.