Computers are electronically noisy.

Electronic eavesdroppers are said to have devices which can accurately read and translate the noise of your computer, display monitor, keyboard input, and printer so that what you see, type or print is what they get.

To eliminate much of this vulnerability to electronic surveillance, I suggest you do your computer work in a large steel lined -including the floor- enclosure.

Inexpensive steel garden sheds are marginally acceptable for this purpose - you'll have to fabricate the sheet metal floor.

If portable radios and TVs can't receive a signal when in the metal enclosure its likely you're secure from this type of eavesdropping.

By the way, if your computer equipment has infrared coupling, don't use it. Mask off all such sending ports.

Your making life extremely easy for the villain with a sub miniature transmitter modulated by a sub miniature infrared sensor.

Are you going to notice a pinhole somewhere in the joints of your metal  shed?

You might take your protection a step further by electrically and electronically isolating your computer equipment's AC power line from other AC power lines.

This can be inexpensively - but noisily - accomplished by using off the shelf equipment: an electric motor, a car alternator or generator, car batteries, fan belts, pulleys and a 12 VDC to 115 VAC power converter.

An outside AC line supplies the electric motor which is mechanically coupled to the alternator.

The alternator supplies current at 12 volts to the car storage batteries and the power converter.

The power converter supplies the isolated 115 volt power line for your computer equipment.

Though not extremely efficient, this type of system will keep your computer equipment's electronic noise from easily accessed outside power lines.

It also protects your equipment from power surges and outages.

Remember, a well trained electronic technician's "just unintelligible electronic noise" is likely a technically sophisticated eavesdroppers "money making information".

Remove all communication gear from your metal lined working place; i.e. radios, cell phones, phones, modems, phone lines etc..

These give the interloper a ready and easy means to transfer vast quantities of data in negligible time and can be the location of a menagerie of eavesdropping devices.

The existence of such equipment in your work area can negate a large part of your security measures.

With explicit information about what type of computer equipment you're using, the espionage sleazoid with one incursion into your work area can  replace your computer equipment with equipment identical in appearance but including circuitry and programming that permit easy and continuous access to your files and computer activities without your knowledge.

I wonder if some computer equipment sold in this country might have one or more of these facilities included in their manufacture albeit in a dormant state, perhaps to guarantee the compatibility of the equipment with future, yet to be marketed computer accessories and systems.

Your modem may be shut off - its switch off and all of its lights off - but it might still be reading and transferring your files or computer activity along its attached phone line.

You can pull its power plug and it'll likely continue chugging along using power from the phone line or a miniature photoelectric cell.

Only a few milliwatts of power are required for this type of electronic circuitry.

To circumvent the possible discovery of a telephone line tap or field coupling, the phone line might be used only as an antenna for a RF transmitter hidden away on the modem's circuit board.

If a modem is one of your essential communication tools, use an additional computer located outside of your secure working area and encrypt all sensitive material before its electronic transfer.