Two weeks ago in this space I set out to convince you that you are making a serious mistake if your personal computer doesn't have a SideKick -- that is, if you don't have a "desktop" utility program like the famous SideKick from Borland International.

As I noted, these programs -- which use your computer to create electronic versions of standard desktop tools like your memo pad, appointment calendar, Rolodex, calculator, and alarm clock -- so greatly enhance operations in any office that you'd be crazy not to have one.

I trust that you succumbed to my efforts at persuasion and are now eager to rush out and get one of these programs. Which leaves only one simple question: Which one?

In the 18 months since SideKick appeared, giving birth to this new "desktop" category of software, there's been a flood of SideKick clones. I've been test-driving some of them.

There are so many "desktop" programs around now that this is definitely a buyer's market. So I set some fairly tough conditions for the programs I chose. I wouldn't consider any program costing more than $50 (via mail-order, that is, but this is precisely the kind of item it makes sense to buy via mail-order). I wouldn't look at one that is copy-protected, because you want to copy your "desktop" program onto your standard boot-up disc and load it automatically whenever you turn on your computer.

I insisted that the program respond instantly when called upon, with no waiting for it to load into memory. I demanded something easy -- a program I could use after one quick glance at the manual.

In a sense, you could say I was identifying the original SideKick (Borland International, 41134 Scotts Valley Drive, Scotts Valley, Calif. 95066), which meets all those tests. SideKick provides an excellent calculator, a fine notepad, an appointment calendar, a speedy electronic Rolodex and some other goodies. They all work perfectly at high speed, and the commands seem almost intuitive -- you hit the key that seems right for what you want to do, and most of the time it is the right key.

I find few faults with what SideKick offers, but I do have some gripes. There's no on-screen clock or alarm function. The calendar shows one month or one day at a time; it's difficult to get a fix on a week's worth of appointments.

In contrast, the SideKick clone called Pop-Up DeskSet (Bellsoft Inc., Bellevue, Wash. 98004) offers just about everything you could dream of in one of these programs: all the standard tools, plus a marvelous command utility called "PopAny" that lets you temporarily exit any program, run any DOS command (Format, Copy, Erase, etc.) or any other program, and then come back instantly to the program you were using.

The Pop-Up set includes a three-month-scan calendar plus a full-screen one-month calendar that will let you get a quick glimpse of your schedule for the next few days or weeks. The program's designers, however, did not provide enough room to type in a complete appointment. And the Pop-up calculator is harder to use than SideKick's.

One of the more intriguing clones is Homebase (Amber Systems, 1171 S. Saratoga-Sunnyvale Rd., San Jose, Calif. 95129). It's got the standard desktop tools plus a built-in, instantly available data-base. This program works fairly well and comes with an engagingly quirky manual. But because of the structure of its various files, it's awkward to use unless you have a hard disc.

A brand-new clone called PC-Desk (Software Studios, 8516 Sugarbush, Annandale, Va. 22003) offers the usual variety of tools and the best appointment calendar. But there are still glitches in this program; the early version I used kept hanging up with the mysterious error message "Device Timeout in Line 740."

Osborne/Kaypro users will be delighted to know about Presto (Spectre, 22458 Ventura Blvd., Woodland Hills, Calif. 91364), which provides instantly available calculator, calendar, notepad, and screen-print functions for CP/M computers.

Among all these possibilities, I think I'll stick with SideKick. There's a one-word explanation: compatibility. These "desktop" programs have to sit constantly in memory so as to be instantly available. That means they are competing for memory space with any other program you are using. When the "desktop" and your application program collide, nothing works. So it's essential that your "desktop" fit with your other software. Since SideKick has become the standard in this category, other software houses pretty much have to see to it that their stuff will work with it.

This is a major concern. Unless you can be assured by the publishers of the program that their software will work with the programs you normally use, your safest bet may be trusty ol' SideKick.