'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 flew in from Katmandu for the holidays. A computer! You plugged it in, punched a few keys and spent the next umpteen hours oblivious to everything except this marvelous toy -- opps, tool.

But now what? Once you're pas the first night's playing around and you've started to sense the fantastic feeling of power and a control a computer provides, here's a suggestion on what to do with your computer the second day: Torture it.

Everyone who gets a new computer should spend some time right away subjecting the machine to assorted forms of torture (If that word seems too violent for the season, we can substitute the phrase "diagnostic testing.") Run your computer through its paces, thousands and thousands of times, just to make sure everything works.

When you get a new computer, it's covered by a warranty that probably lasts 90 days or so. If your machine is going to have a glitch, it would be nice for it to appear during the warranty period, when repairs are free. That's why I recommend "diagnostic testing -- the idea is to force potential problems to the surface right away.

I make the same suggestion in this space each Christmas. It has paid off for readers like E. Peter Robare, of La Grange, Ky. Robare got a Commodore for Christmas last year and immediately started running the torture tests. "Sometime into the third hour," he wrote me, "the screen went 'garbage' and a short time later, completely blank."

Robare took his computer back to the dealer during the warranty period. Not only did he get a new Commodore, but he also got a $38 refund because the price had fallen that much since his had been purchased. The new machine passed the torture tests and is still running fine.

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 computer a program like this one in BASIC:

10Z=X*X*X

20 PRINT X,Z

30=X+1

40 GO TO 10

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

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

You should also test the input-output circuitry to make sure that what you type in gets to the mocroprocessor and shows up on the screen. One way to do this is to find a complaint 10-year-old, if there is such a thing, and have him bang away at the keys for an hour or so.

But why make a human perform this simple, repetitive task? This kind of mindless chore is the reason we have computers. With a short program based on the following sequence, you can automatically run through every character your computer knows:

10 FOR A=1 TO 127

20 PRINT CHR$(A): NEXT (Depending on which computer you have, you can increase the last number in line 10 to 255. If you change the "PRINT " command in line 20 to "LPRINT," you'll be able to torture-test your printer, too.)

The single componment of a computer that's probably most vulnerable to breakdowns is the disk drive. It's really just a small record player; unlike the logic circuitry, it has moving parts that can go on the fritz at any time. Here's a BASIC program that will test the drive by spinning the disk and reading a file as many times as you like (25 times in our example):

10 FOR A=TO 25

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

CLOSE #1: NEXT (This program assumes you have a file called "FILE DAT," or whatever name you use, on the disk.)

Some computers have diagnostic programs included on the operating system disk or listed in the user's manual. Typically, these programs will test every memory address (by writing in a number and reading it out again) or will check the rotation rate of your disk drive. Run these, too, right at the start.

If your computer flunks any of these torture tests, gets it back to the shop while it is still under warranty. That way you can be sure the machine won't start to torture you as soon as the 90 days are up.