Personal computers don't cost nearly as much as they used to, but they still cost plenty. And if you aren't careful, you can spend an additional bundle on software that ends up as shelfware -- stuff you don't like, don't use and can't get rid of. Fortunately, there's a way to avoid this, at least in the world of IBM computers and their clones.

It's called "shareware" -- software that is copyrighted but not copy-protected, that users are encouraged to try, copy and pass around. If you like and use a shareware program, the author asks that you send a registration fee. In return, you usually get a printed manual, telephone support and free updates.

Instead of fighting the inherent copyability of software, shareware authors exploit it -- indeed, depend on it -- to get their products distributed. You will find shareware on computer bulletin boards, in user group libraries or for sale at a nominal charge from clearing houses, such as PC-SIG in Sunnyvale, Calif.

Perhaps the best known of shareware packages is Jim Button's PC-File, a program that has 45,000 registered users and is now in an update that includes many of the functions of big commercial programs with list prices of $300-$400. Yet PC-File is perhaps best known for its ease of use, something that cannot be said for many of its larger and more expensive competitors.

The registration fee for PC-File is $69.95, and Button's firm (Buttonware, P.O. Box 5786, Bellevue, Wash. 98006, 1-800-J-BUTTON) has annual revenue in excess of $2 million. The company also distributes a range of other shareware programs, including a word processor, spreadsheet and communications program. All have their following, but none is as successful as PC-File.

Second only to PC-File in popularity is Bob Wallace's PC-Write, a word processor whose speed is second to none and whose power rivals that of such commercial favorites as WordPerfect, Xywrite and Multimate. The latest update of PC-Write has a spelling checker, 45 help screens and a powerful macro capability that is remarkably easy to use. The program has numerous endearing small features, such as a word counter that also gives a byte, letter and character count. It also computes the average word length.

Like everything else about PC-Write, the count feature is blindingly fast. PC-Write specifically supports virtually every known printer and will run on some (though not all) of the least-compatible IBM clones. You can get PC-Write for $16 from Quicksoft, 219 First N. 224, Seattle, Wash. 98109, or by calling 206-282-0452. A fully registered copy with a 336-page bound manual is $89 and includes two free updates, plus telephone support. And if you register, your copy will have a number.

And if someone registers from your copy (and says so), you get a $25 commission. That incentive helps explain why the program has 24,000 registered users and Quicksoft's annual receipts exceed $1.5 million.

Number three on the shareware hit parade is the modem communications program Procomm, from young programmers Bruce Barkelew and Tom Smith of Columbia, Mo. Procomm is considered by many to be the best communications program available at any price. Major modem manufacturers, including Multitech, Omnitel and Practical Peripherals, are seeking rights to bundle it with their products. The program is written in C language, which makes for fast execution, and it is loaded with bells and whistles, including seven file-transfer protocols. It has a powerful, yet uncomplicated, script feature that makes it easy to create automatic log-on files for connecting with mainframe systems or other microcomputers.

Registration is $25, or $50 if you want a printed copy of the instructions. The full instructions are included on the disk. Barkelew and Smith say their company, Datastorm Technologies (Box 1471, Columbia, Mo. 65205, 314-449-7012), has nine employes and has been adding about one staffer a month for a year. Receipts are now running about $200,000 a month.

While these three programs are the most successful, there is plenty of other good shareware. Almost giving the product away may seem an odd way to make money, but it is working, at least for some. As PC-Write's Wallace explains, he loses nothing when someone who would never have bought his program uses it without paying, since there are no distribution costs. The only possible loss comes from a person who would have bought the program, but who gets it for free and doesn't pay the registration fee.

Wallace and the others are betting there aren't many users like that. So far, they're winning.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