Pre-owned computers can be tested in a matter of hours, following procedures Alexander Randall V, cofounder of Boston Computer Exchange, recommends in Alex Randall's Used Computer Handbook (Microsoft Press, 1990, $14.95). Some examples:

* Open the box. A layer of dust would be discouraging.

* Power-up from a floppy disk in the A drive. If the system fails, its floppy-disk controller card, floppy-disk drive, or motherboard may be defective.

* Try booting up from the hard disk. A failure suggests the hard disk has been improperly formatted or is missing essential start-up files, the hard-disk controller card is defective, the hard disk is defective, or the motherboard is defective.

* Check the memory. Make sure the amount of RAM displayed on the monitor after the initial memory check jibes with what it's supposed to have.

* Format a floppy disk. If the format fails, try a fresh disk. Successive failures suggest the drive is defective.

* Try copying files from one drive to another. Repeated failures indicate a defective drive. If you're successful, run a disk comparison to make sure the files were copied accurately.

* Examine the monitor. Turn up the brightness and contrast controls to check for phosphor "burns." Determine if characters are out of proportion. Make sure the image doesn't waver.

* Check the keyboard. Do the characters on the screen match each character on the keyboard? Test the cable by gently tugging on it.

* Don't ignore the fan. If, when you turn the computer on or off, the fan sounds like a helicopter, it's probably ailing, says Randall. It can be replaced, but it may have damaged the heat-sensitive electronics inside. -- Robert A. Mamis