You can tell the Christmas shopping season is here: Tandy Corp. is flooding the television airwaves with ads for its low-priced computers. By and large, Tandy computers are good values, especially for the home or small-business user. But there are some things you should know before you buy one.

Tandy's 1000-series IBM-compatibles now dominate the low end of the PC market. The company has moved aggressively into the opening created by IBM's failure with the PCjr and Big Blue's decision since then to leave that market to others. IBM's new model 25 is an entry of sorts, but at $1,350 (list) for a black-and-white monochrome system with one disk drive, it is no bargain.

Compare that with Tandy's 1000 HX, at $599 without monitor for a one-drive system with 256K of memory. With a monochrome monitor, it's $728. Add a second disk drive and expand the memory to 640K, and it's about $1,100. With a decent color monitor, it's about $1,300, still below the IBM monochrome system. The 1000 HX comes with a 720K 3 1/2-inch disk drive, but you can make your second drive either another 3 1/2 or a 360K 5 1/4.

The HX has an unusual feature. The software operating system, MS-DOS, is on a built-in chip, so the computer starts or "boots up" very fast with no need of a special "system disk." This is a feature many users weary of rummaging for their MS-DOS startup disk will appreciate. But you can use a version of MS-DOS other than the one built in, by loading it from a disk.

Tandy has a similar model, the 1000 EX with a 5 1/4 internal drive and no MS-DOS chip. It is being discontinued and now sells for $499 for a stripped down, no-monitor 256K model. Tandy will continue to supply parts and service. Both the HX and EX are single-unit computers with keyboard attached.

There is also a more conventional two-piece model, the 1000 SX, with detached keyboard and a larger system unit with room for five expansion slots. With 384K, one 5 1/4-inch disk drive and no monitor it sells for $849.

All these Tandy computers have a two-speed processor that allows them to run faster than the original IBM PC, or at the same speed. All come with a Tandy six-in-one software package called Deskmate, which includes a word processor, database program, modem communications software and other utilities. It isn't fancy, but it's functional and many people will find it all they need.

With more than 7,000 Radio Shack stores and Tandy Computer centers nationwide, the company has a retail and service organization no one can match. No matter where you are, you can usually find a Radio Shack outlet where someone can help you.

Unfortunately, the level of computer expertise varies widely from store to store, and that's not Tandy's only drawback. The claims of added performance from the two-speed processor are exaggerated. At its faster speed, the Tandy 1000 models just barely outrun an IBM PC running at normal speed.

There is one exception, the Tandy 1000 TX, built around the Intel 80286 microprocessor. This is the chip used by IBM in its AT and its PS2 models 50 and 60, and in countless AT clones. These 80286 machines run software written for the original IBM PC, but at much greater speed. At $999 with 640K and no monitor, the Tandy 1000 TX is the least expensive name-brand 80286 machine you can buy. But it is not a true AT-compatible because it will not run expansion boards designed for the AT. It is simply a fast PC-compatible.

The worst single feature of the 1000 series is the keyboard. It feels all right, but some of the key placements (especially of the Alt, Delete and Insert keys) are annoyingly unique. They can make you feel as if you're learning your software all over again. You can't substitute a different keyboard with the EX and HX because the keyboard is part of the unit.

And the 1000 series, especially the EX and HX, will not accept many hardware enhancements designed for the IBM originals. That often means you must buy from Tandy, whose prices on expansion products are high. For example, a memory upgrade from 256K to 640K is $210 from Tandy, as little as $55 from other vendors.

Still, the Tandy 1000 series is a good, safe buy. And there's one more Tandy computer worth mentioning -- the 102 portable, successor to the model 100, the original laptop. At 3 pounds and $449, with a built-in word processor and modem, it may be the most powerful computer per ounce ever made.

Its eight-line, 40-character LCD screen leaves much to the imagination, and its processing speed is slow. But startup is instantaneous and the 102 is simple to use. Third-party companies make useful software enhancements that come on tiny ROM chips. It only has 24K, expandable to 32K, but there's a portable 200K disk drive ($200) if you need more storage.

Brit Hume is a contributor to the Washington Post Writers Group. Hume is an ABC News Capitol Hill correspondent and the founding editor of a computer newsletter.