For the reader of computer magazines, the flashy, enticing ads from the mail-order houses are almost irresistible, with their promises of low prices, fast delivery and full warranties. But beware, gentle reader! Those mail-order ads are the modern equivalent of the Sirens that tempted Ulysses and his crew in the 12th book of the Odyssey: sweetly seductive, but potentially disastrous.

There are times when it makes sense to buy computer gear via mail order. If you have confidence in your knowledge of hardware and software (in other words, if you know exactly what you need), if you're certain you won't need help installing or using the item, if you have confidence in the manufacturer and you know something about the reliability of the mail-order house, then you can save money buying through mail order.

Let's say, for example, that you need a word-processing program for your computer at home. Let's assume that you have already used WordPerfect at the office. Let's assume further that you know for sure the program will run unimpaired on your machine at home and that you don't need anybody to advise you on installing or running the program. If all those conditions are met, buy the program through mail order; you'll get about $100 off the suggested price and $50 off the price at a discount retail store.

But if you have any uncertainty about the hardware or software you need, you'll be happier if you resist the temptations of the mail-order sirens. You pay more, as a general rule, at a retail store, but you get a lot in return.

A retail salesperson near your house can make sure that you buy only the gear that will work with your system. He or she can let you take a test drive in the store so you can find out if the new Super-Turbo monitor or modem, or whatever, is really as good as you think it is. And after you get the new peripheral or program home and you can't decipher the first sentence of the instructions, it's pleasant to know that there's a real human being (not a detached voice on the telephone) within driving distance who can set things right.

The time when you most need a retailer, by the way, is when the retail premium is highest -- that is, when you're buying an expensive piece of hardware or a complete system. You can save hundreds of dollars buying a new computer system from a mail-order house. But what if this lavish new machine doesn't work? What if you can't get your most crucial piece of software to run, or your modem won't fit in any slot? That's when you need a retailer by your side -- and frankly, the peace of mind you'll get is worth the extra $300 it costs to buy at a store.

I have bought a great deal of software via mail order over the years. For the most part, the results have been satisfactory. But along the way, I've acquired a small collection of programs that didn't do the job I expected. Had I resisted the temptations of mail order and gone down to the store instead, I could have tried out the program ahead of time -- and saved a lot of money.

On the hardware side, you'd need a strong stomach to listen to all my horror stories. A month ago, for instance, I "saved" a bunch of money by buying a new monitor from the big Hawthorne, Calif., mail-order house called Jade Computer. In less than a week, Jade delivered the exact model advertised. The monitor ran beautifully when I hooked it up. It continued to work perfectly for a full two weeks, at which point something snapped inside and the whole display shrunk down into a single whisker-thin stripe of light in the middle of the screen.

I called Jade. The operator was nice enough, but her message wasn't. We were required to find a secure box, deliver the monitor to a shipping service, and send it back to California (at our expense, of course). Any damage in transit would be our fault.

At some point, a Jade technician would perform an autopsy on the unit and tell me whether I would get my money back. In the meantime, I have no display monitor -- a shortcoming that can really cut into your enjoyment of a computer.

None of this is Jade's fault; the company has done everything according to its written policies. This kind of problem is inherent in the process of buying complex high-tech gear through mail order. For the buyer, mail order sometimes can be a boon. But it can also be a decidedly risky business.