BOSTON (CNN/Money) - When Microsoft announces that it's entering a new market, investors in companies already in that market almost always panic. And with good reason: Microsoft has a long track record of annihilating competitors.

That was undoubtedly the thinking of McAfee (Research) and Symantec (Research) investors in the days before Microsoft (Research) formally announced its intentions to enter the PC security space. When the Redmond specter appeared further off than the Street had anticipated, the relief of those same investors put some pop back into the stocks.

But now, in the wake of Microsoft's announcement, both stocks are drifting down once again. For Symantec investors, the downward movement is a mistake and signals a possible buying opportunity -- with a couple of caveats we'll discuss later.

For McAfee, however, the trouble could be just beginning. Investors will learn how much trouble tomorrow when McAfee announces its fourth-quarter earnings. They'll want to find out how McAfee's consumer revenue is faring to see what kind of inroads, if any, Microsoft's product has already made.

Let's focus on the leader in this market, Symantec, and why it is in a better position than McAfee to fend off Microsoft. In its last quarterly filing, Symantec reported that it earned 52 percent of its revenue from the consumer market, thanks to a successful subscription model in which consumers get regular antivirus updates.

The wonderful part of that model: "It's pure margin," says Gene Munster, an analyst with Piper Jaffray. Analysts estimate that 60 to 70 percent of Symantec's income comes from the consumer segment.

Microsoft's first foray into the security space has already been unveiled: It's a consumer antispyware product that is expected to come out of beta testing very soon. Not surprisingly to anyone who's followed Microsoft's tactics in new markets, the company is giving away the product for free.

"Wait a minute," I can hear you say. "With most of Symantec's earnings coming from the consumer space, isn't it vulnerable to Microsoft's encroachment?"

Don't be so sure about that. First of all, Microsoft isn't Superman. There are a number of markets it has tried and failed to take. Intuit's Quicken, Sony's PlayStation, and Liberate have all held their ground.

"So what?" you might say. Well, Symantec is run as well as, if not better than, those companies. CEO John Thompson knows how to lead and how to execute a plan. The company is full of excellent marketers who have methodically killed off competitors.

It can hardly take much effort by Symantec to convince consumers who are worried about viruses that they need an independent virus block, not a product from the company that created the vulnerabilities in the first place -- and that charged them for the buggy stuff.

Symantec is in the midst of acquiring Veritas Software, which will boost its enterprise offerings significantly. (It handles acquisitions superbly; it's done dozens of them in the past three years.)

This makes it less dependent on the consumer side. Symantec's enterprise revenue is expected to increase from 35 percent to 66 percent, and its total annual revenue from about $2 billion to $4 billion. (McAfee, on the other hand, doesn't have a plan to bolster its enterprise products and services.)

Microsoft hasn't laid out a specific road map for its strategy in this market. And for the foreseeable future, the enterprise territory may very well be Microsoft-proof, because there's a legitimate anti-Microsoft attitude in the enterprise space where security is concerned.

For many enterprises Microsoft has been part of the problem with regards to security. Accepting it as part of the solution will be a pretty big challenge.

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