I don't often review IBM compatible computers. There are so many on the market and, in many cases, it's almost impossible to tell them apart. But once in a while I come across a machine that distinguishes itself. Positive Corp.'s 486-25 qualifies because it delivers high performance at a modest price.

At $2,999, the 486-25 isn't inexpensive. It is an excellent value for people who need a high-performance PC. The machine is equipped with an Intel 486 central processing unit that runs at 25 megahertz. This is the first fully equipped 486 machine that I've seen for less than $3,000.

The basic system comes with 2 megabytes of memory and a 106-megabyte hard disk. It has two floppy drives (1.2 megabytes and 1.44 megabytes), a mouse, a color VGA monitor (1,024 by 768 pixel) and a keyboard. A 128-kilobyte memory cache improves performance by storing the most recently used instructions in a special bank of high-speed memory. There are two serial ports and a parallel port. It also comes with the MS-DOS 4.01 operating system, Windows 3.0 graphical user interface and working models of several Windows programs.

The company's distribution strategy is also unique. The machine is available at Price Club and Sam's warehouse membership department stores. It is also available by mail order for about $200 more. Warehouse stores are not known for their customer service, but Positive offers its own toll-free support line as well as a one-year warranty and free on-site repairs for the first 30 days.

I placed the Positive 486 next to my 25 megahertz 386 system to compare the performances. My 386 is a fast machine, but when it comes to raw processing power, the Positive 486 is more than twice as fast. One popular performance test, a program called System Information from Peter Norton Computing, clocked the machine's central-processing unit at 54 times the speed of the original IBM PC, 12 times the speed of the original IBM AT and more than 1.5 times the speed of Compaq's 33 megahertz 386 machine.

But processor performance does not necessarily translate into faster computing. Using a 486 machine is like driving a high-performance sports car. It makes a difference on a race track, but not in normal driving.

There are some applications where speed is critical. Some computer-aided design projects, for example, can take several minutes to re-draw a screen on a complicated drawing. There are scientific programs that take hours to run. Other uses for fast machines include software development, desktop publishing and multi-user database management systems. Finally, there are a lot of people who simply enjoy using a computer that's lightning fast, whether they need it or not. Satisfying that craving is now a lot more affordable.

In addition to being fast, the machine is very easy to set up and use. It usually takes me more than an hour to set up a new machine by the time I plug everything together, format the hard disk and copy MS-DOS from the floppies to the hard disk. This time it took less than 10 minutes. That's because the hard disk comes pre-formatted with MS-DOS and Windows already installed. You plug in the monitor, keyboard and mouse and just turn it on. A few seconds later you're in Windows 3.0, ready to click on the icon of whatever program you want to run. The sample software includes working models of several Microsoft Windows's programs including Word, Excel, PowerPoint and Project. These are not the official release copies, but disabled models that give you a chance to experiment with the programs. There is also a real copy of Microsoft's Productivity Pack programs that helps you learn to use Windows 3.0.

Unfortunately, the machine does not come with any floppy disks. The manual recommends that you copy the software from the hard disk to floppies, but that's not always easy, especially with Microsoft Windows. But, considering the cost of the system, you could go out and buy MS-DOS and Windows and still be ahead.

Positive offers less-expensive machines with 386 and 286 processors. The company is located in Chatsworth, Calif., and can be reached at 818-341-5400. Toll-free numbers are 1-800-252-6345 within California and 1-800-452-6345 outside California.

Readers' comments are welcomed, but the author cannot respond individually to letters. Write to Lawrence J. Magid, P.O. Box 620477, Woodside, Calif. 94062, or contact the L. Magid account on the MCI electronic mail system.