Twas the night after Christmas -- sometime in the wee, small hours before dawn -- and all through the house, not a creature was stirring except you and the electronic mouse attached to the personal computer you found under the tree. You got a computer!

If you're like most new computer owners, you instantly forgot about such trifling distractions as your spouse, your kids and the favorite uncle who had flown in from Katmandu for the holidays. A computer! You plugged it in, punched a few keys, and spent the next umpteen hours utterly mesmerized by this new toy (oops -- I mean "tool," of course).

But now what? Once you've spent that first long night getting a feel for the fantastic power a computer provides, what should you do next? Here's a modest suggestion on what to do with your computer the second day: Torture it.

Loyal readers may recognize that this is the same advice I gave at this time a year ago. But this column runs in more newspapers than it did then. Moreover, several veteran readers have asked to see "the torture column" once more. So, here I go again.

Everyone who gets a new computer should spend some time subjecting the machine to assorted forms of torture. (If that word seems excessively violent for the holiday season, you can substitute the phrase "diagnostic testing.") You should run that PC through its paces, thousands and thousands of times, to make sure everything works.

When you get a new computer, its glitches are covered by a warranty, which probably lasts 90 days. If your machine is ever going to have trouble, it would be nice to have it happen during the warranty period, when repairs are free.

That's what "diagnostic testing" is all about. The idea is to force all potential problems to the surface before that warranty expires.

This makes particular sense with computers and other digital devices. The innards of all these microelectronic marvels -- the silicon chips -- have no moving parts (except the electrons zipping around the circuitry). That means a chip will almost never break; if it's good when you first turn on your computer, it's good forever.

There are some diagnostic programs on the market, with prices ranging up to $100. For the most part, I consider this software a waste of money. The fact is, you can do a lot of torture-testing on your own.

The first thing you should do with a new computer is turn it on -- and keep it on, nonstop, for three days or more.

Then you should start computing -- and keep on computing, nonstop, for a few hours or days. You might feed your machine a program like this one in BASIC:

10 Z=X*X*X

20 PRINT X,Z

30 X=X+1

40 GOTO 10

The experts will tell you that this bit of code (it computes the cube of every integer from zero on up) is an "endless loop," which is verboten at programming school. But it's a nice little endless torture, and it will keep your computer humming productively away all night. If your computer is still getting the cube right after a few million passes, you can be fairly sure the logic chips in your computer are okay.

If you're using a cheap TV set for display, this program could burn a spot on the screen, so don't run it for more than an hour or so unless you have a real computer monitor.

You should also do some diagnostics on the input/output circuitry, to make sure what you type into the computer gets to the microprocessor and shows up on the screen. One way to do this is to find a compliant 10-year-old, if there is such a thing, and have him bang away at the keys for an hour or so. (Don't worry -- you can never damage a computer by typing on its keys.)

But most 10-year-olds are too smart to put up with such a simple, repetitive job for long. This kind of mindless task is what we have computers for. With a short program based on the following sequence, you can run automatically through every character your computer knows: 10 FOR A=1 TO 127

20 PRINT CHR:NEXT

If you change the "PRINT" command to "LPRINT" in line 20, you'll be able to torture-test your printer, too.

You'll also want to test your machine's memory. Many computers have built-in diagnostic programs -- usually found in the tiny print at the back of the manual -- that will move systematically through each memory address, writing in a number and reading it out again to test for accuracy. You can do the same thing in BASIC with a series of POKE and PEEK commands.

The single component of a computer that's probably most vulnerable to breakdown is the disk drive. It's really just a small record player with some moving parts that can go on the fritz at any time. Here's a BASIC program that will spin the disk and read a file as many times as you like (25 times in our example):

10 FOR A=1 TO 25

20 OPEN "I", #1, "FILE.DAT"

30 CLOSE #1: NEXT

(This program assumes you have a program called "file.dat" -- or whatever name you use -- on the disk.)

Some computers also have a program, back in the small print, to test a drive's rotation rate. Run that one, too, right at the start.

If your computer flunks any of these tests, get it back to the shop lickety-split -- while that warranty is in effect. That way you can be sure that your computer doesn't start to torture you as soon as the 90 days are up.