'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 new toy (oops -- I mean "tool," of course).

But now what? Once you're past the first night's playing around and you've started to sense the fantastic feeling of power and control a computer provides, what do you do next? 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, substitute the phrase, "diagnostic testing"). You should run your computer through its paces, thousands and thousands of times, just to make sure everything works.

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

I made the same suggestion in this space last Christmas. It paid off for E. Peter Robare, of La Grange, Ky., who reads this column in one of America's great newspapers, the Louisville Courier-Journal. He got a Commodore 64 for Christmas last year and immediately starting running my torture tests. "Sometime into the third hour," he wrote me later, "the screen went 'garbage' and a short time later, completely blank."

Mr. Robare took his computer back to the dealer within the warranty period. Not only did he get a new Commodore, but he got a $38 refund, because the price had fallen that much in the brief time since his had been purchased. The new machine passed my 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, just as Mr. Robare did. You might feed your computer 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 in programming school. But it's a nice little endless torture, and will keep your computer humming productively away 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 a cheap TV 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 real computer monitor. You should also test the input-output circuitry to make sure what you type in 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.

But why make a human perform this simple, repetitive task? This kind of mindless chore 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

(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 component of a computer that's probably most vulnerable to breakdowns is the disk drive. 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=1 TO 25

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

30 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 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, get it back to the shop while the warranty lasts. That way, you can be sure the machine won't start to torture you as soon as the 90 days are up.