The best bargain in the personal computer business right now -- and arguably the best bargain in personal computer history -- can be found in Radio Shack stores that still are selling the PC industry's most famous "lap-top" computer, the TRS-80 Model 100.

Radio Shack's parent company, Tandy, has discontinued the Model 100 so that it can focus its effort on two fine successor lap-tops, the slightly larger Model 200 and the slightly smaller Model 102. But there are still thousands of Model 100s in the distribution network. Radio Shack dealers are unloading them cheap; the official nationwide closeout price Model 100 is $299 (for the computer with 32K bytes of memory), but I've heard of stores offering the machine for prices as low as $199.

If you are walking past a Radio Shack and you see a Model 100 on sale for a price like that, there's only thing to do: buy that baby, and then run home and congratulate yourself for knowing a terrific deal when you see one.

Even in an era when miracles of miniaturization seem commonplace, the Model 100 packs a remarkable amount of computing power into a minute package (the computer is approximately the size and weight of a medium-sized city's telephone book). During the past three years, it has become the tool of choice for tens of thousands of people who have to do their computing on the fly -- on the airplane, in the back seat of a rental car, wherever traveling professionals find themselves up against a deadline.

Thus, the Model 100 has become as ubiquitous among journalists as the familiar manila reporter's notebook. Traveling musicians use it to bang out new lyrics and keep track of expenses. The Forest Service has bought a batch for researchers in remote areas who are gathering botanical data.

The folks at Radio Shack have done a good job of reaching this natural market among people who need an unobtrusive, but powerful, little computer on the road. I respectfully submit, though, that Radio Shack, like most other retailers of "lap-top" computers, has missed the biggest market of all for this tiny wonder.

The great thing about these little lap-tops is that they can serve as an inexpensive second or third computer for anyone who already is using a desk-top personal computer. To understand this, you should think of the Model 100 as a sort of second keyboard. It's as if you could unhook the keyboard from your desk-top computer and take it with you anywhere, and still do all the work you normally do on your computer. Or, while your kids are playing Alien Zappers on your computer at home, you can sit at the dining room table with your "second keyboard" and do your work without interrupting the kids' game at its crucial moment.

With the proper equipment and software, any lap-top computer can serve this "second keyboard" function, effectively letting you work on your desk-top computer wherever you may be.

There are three different ways to make this work:The simplest (and cheapest) thing to do is to work on your lap-top, with the lap-top's software, and then send (or "upload," to use the tekkies' term) everything you've done to your desk-top computer via a modem-and-telephone hookup. This works very nicely for word-processing files and other simple stuff, but it limits you to the programs available on your lap-top computer. It's more satisfying to get one of those "remote control" programs for your desktop -- that is, software that permits you to call your desk-top computer from any remote computer (including the Model 100 you've brought along on the trip) and run any program as if you were seated at your desk. I have had good success running an IBM-PC through a Radio Shack Model 100 using the program called "Remote" from Microstuf (1000 Holcomb Woods Pkwy., Roswell, Ga., 30076). With that combination, I can run any IBM-PC program and watch the results on the screen of my lap-top. The fanciest, and most effective, way to use a lap-top is to turn it into a traveling facsimile of your desk-top computer. A Seattle outfit called Traveling Software ( 800 343-8080) has a whole catalogue of inexpensive tools to do this. With Traveling Software's help, you can buy a tiny, lightweight disk drive for your lap-top and an operating system (TS-DOS) to run it. The firm also provides software ("LAPDOS") that makes it a snap to transfer (okay, to "upload") data and text files from the little portable drive to the disk drive on your desk-top computer. If you're a serious user of any Radio Shack lap-top (or the work-alike NEC-PC 8201), you owe it to yourself to get the Traveling Software catalogue.

I've recently had the chance to test-drive Radio Shack's replacements for the Model 100, the Model 200 and the snappy new "compact" version, the Model 102. I'll report on these machines in a future column. For now, though, keep your eye alert, and your checkbook ready, for a $200 closeout offer on the Model 100. It's a fantastic deal.