The immense popularity of laptop computers has spawned a lively commerce in software designed principally to do one thing: transfer files created on a laptop system to a desktop computer, and vice versa. It is a simple task, but in the world of software, nothing is ever simple once the competition to have the most features begins.

The simplest way to transfer a file from one computer to another is to copy the file onto a floppy disk and take the disk to the other computer, stick it in the disk drive and take it from there. Alas, this is not always possible. Different computers have different kinds of disk drives.

If your desktop has a 5 1/4-inch floppy drive, for example, and your laptop a 3 1/2-inch, the problem is obvious. The same is true if one computer has a high-capacity floppy drive and the other does not. You can get around that by formatting the transfer disk for lower capacity, but this requires you to deal with the intricacies of the MS-DOS format command. Most people would rather go to the dentist.

So the solution for many users is one of the growing number of programs intended to take the pain out of file transfer. It is the only solution if your laptop is among the crop of notebook-size portables that has a hard disk, but no floppy drive. Indeed, many such models come with this kind of software.

These programs have one thing in common. They use a wire, the technical name of which is a "null modem cable," to hook the two computers together. Most file-transfer software comes with the necessary cable. They connect the computers through their external RS-232, or "serial," port. You don't need to know what these terms mean, only be able to locate them on the back of your systems. They are usually clearly marked.

Most computers have two serial ports. One is internal, for use by an internal modem. The other is the external port. One is called "COM1," the other "COM2." It doesn't matter which is which, but you'll need to know to ensure communication between two computers. Your computer's instructions probably will help with this.

Perhaps the best-known file-transfer program is LapLink, now in version 3, $139.95 list from Traveling Software. Its installation is straightforward. You copy the program to a disk on each of your computers and then, once the two systems are connected, run the program on both. On each display will appear a split screen showing a list of files in the current directory of each. You use the left and right arrow keys to move a wide-bar cursor between the two windows. You use the up and down arrow keys to move among the files in each window.

Across the bottom of the screen is a list of commands: copy, mark, tag, move. You invoke one by hitting a highlighted letter key in each. You can carry out operations with your files from either computer's keyboard. In addition to commands for transferring files, or groups of tagged files, between the systems, there are commands to rename, erase, find and view files. These give LapLink the ability to do a lot of file management chores that may not involve the other computer.

The best thing about LapLink is it's ease of use. You can see the list of files on the directory of the disk of your choice on both computers and move or copy files between them quickly. You barely need the instructions. But the programmers have added a new feature -- remote installation, which is intended to let you install LapLink directly from one computer to another, without use of the installation disks.

This may sound great in ad copy, but it involves some tedious goings on with the MS-DOS MODE and CTTY commands, doesn't always work and generally gets LapLink away from the straightforwardness that is its hallmark. It's not worth the trouble.

A newer entry in the field is Hot Wire, $129 from Datastorm Technologies, the company that produced Procomm, the hands-down best and easiest communications program in the the MS-DOS world. You would expect Hot Wire to be as easy, but it's not. Like LapLink, it lets you look at files on two computers and move them back and forth, but you can't see the files on both systems. Everything is controlled from the screen and keyboard of one.

What's more, you'll need the instructions to figure out how to switch from one disk drive or directory to another. You can get a split-screen view of files on both computers. But even then, you are likely to find the way the program uses the arrow and tab keys to move between windows and among files bewildering.

Hotwire has some advanced features, macros for example, but you may be so daunted by the basic stuff you'll never get around to trying them. Better wait for a new and improved version of this one.

Brit Hume is a contributor to the Washington Post Writers Group. He is chief ABC News White House correspondent and the founding editor of a computer newsletter.