This article is from the PGP FAQ, by Jeff Licquia firstname.lastname@example.org with numerous contributions by others.
TEMPEST is a standard for electromagnetic shielding for computer
equipment. It was created in response to the fact that information can
be read from computer radiation (e.g., from a CRT) at quite a distance
and with little effort. Needless to say, encryption doesn't do much
good if the cleartext is available this way. The typical home
computer WOULD fail ALL of the TEMPEST standards by a long shot. So,
if you are doing anything illegal, don't expect PGP or any other
encryption program to save you. The government could just set up a
monitoring van outside your home and read everything that you are
doing on your computer.
Short of shelling out the ten thousand dollars or so that it would
take to properly shield your computer, a good second choice might be a
laptop computer running on batteries. No emissions would be fed back
into the power lines, and the amount of power being fed to the display
and being consumed by the computer is much less than the typical home
computer and CRT. This provides a much weaker RF field for snoopers to
monitor. It still isn't safe, just safer. In addition, a laptop
computer has the advantage of not being anchored to one location.
Anyone trying to monitor your emissions would have to follow you
around, maybe making themselves a little more obvious. I must
emphasize again that a laptop still is NOT safe from a tempest
standpoint, just safer than the standard personal computer.