Achilles had his heel. Shaq has the free-throw line. Superman has his Kryptonite.

Whether we're mythological, gargantuan, or superhero, we all have our weaknesses. Me? Like Homer Simpson, it's the glazed doughnut. Curses, you sugary decadence!

What about your portfolio? Have you ever combed through it to see where the weak spots are? Oh, believe me, they're there. If you don't see them, you just have to look harder. Either that, or take a few steps back until you see the blemishes that come from a less adoring glimpse as a more objective investor.

Superman is able to deflect bullets and fly through the air, but our fictional hero curls up like a helpless baby when exposed to kryptonite. It's not a problem, of course, as long as you avoid the stuff like poison. Either that, or you just make sure that your enemies don't know your soft spot.

Let's go over a few superhero stocks that are flying high at the moment. Each one has a chunk of kryptonite with its name on it that could send it crashing down to Earth. That doesn't mean that they will crash and burn. Not by a long shot. So let's also explore what each company has to do to make sure that it steers clear of the bad stuff to continue its "Man of Steel" powerful growth trajectory.

Most people love Google. Competitors don't, but that's not the real potential pitfall here. Even as rivals ramp up their paid-search products or fortify their portals, Google's search engine is too strong to topple, and its collection of advertisers and network of third-party publishers is too substantial to overcome on mere might alone.

Kryptonite? Click fraud. The company dismisses the claims. It points to studies showing that click fraud isn't as prevalent as some may fear. The problem is that other studies show otherwise. The bearish thesis says that as advertisers begin to account for pilfered clicks surrendered under fraudulent pretenses, they will bid lower or curb their interactive ad spending substantially.

Antidote? Hogwash. Click fraud is going to be a concern, but it's already baked in to the bidding-war process. The beautiful thing about paid search is that a campaign is instantly accountable. Regardless of the amount of clicks that wind up being bogus, the free market has already dictated that online advertisers are getting a great return on their investment. Why else would they be flocking to the medium in general, and Google in particular, in such great numbers? Google's technology will perpetually improve to discount the shenanigans, but even if it's a problem that will never go away, sponsors know that they are getting a lot of bang for their buck that way.

Nearly 5 million of us across the nation are now smitten by red mailers containing DVD rentals. The Motley Fool Stock Advisor newsletter recommendation has seen its shares triple since bottoming out last year. Competition from Blockbuster (NYSE:BBI) and the threat of (NASDAQ:AMZN) entering the market once weighed heavily on the company, but it's clear that Blockbuster is struggling and the substantial lead in this niche makes an attempt by Amazon less worrisome.

Kryptonite? Video on demand. Every passing year, our broadband-enabled future brings us one step closer to making digital delivery a mainstream reality. In many ways, it's already here. The lack of available titles with pay-per-view and chunky download times with PC-based models are roadblocks at the moment, but time should help lower those evolutionary hurdles.

Antidote? Video on demand still can't duplicate the art of DVD rentals. The special features and outtakes and the ability to play a film with or without directorial comments are some of the reasons why DVD has become the platform of choice, while most video-on-demand offerings are too linear in the old-school VHS ways. Netflix also has the established distribution center in place that will help it evolve into areas like video-game or light media rentals, in which the technology is further advanced for digital distribution but nearly impossible to duplicate in its entirety.

Apple Computer (NASDAQ:AAPL)
The Mac daddy has bounced back in a major way on this side of the millennium, thanks to the breakthrough iPod line and its innovative computing products. There was always the fear that Microsoft (NASDAQ:MSFT) was too strong in the operating software space to allow Apple a chance to make a dent with its proprietary system, but it has now made peace with Mr. Softy, and its newest Macs are bilingual -- they speak Microsoft's language if you let 'em.

Kryptonite? Yesterday's slide amid concerns that there may be a delay in the rollout of new nano and wide-screen iPods is a warning to investors that Apple is overly dependent on the success of its portable media players. The "halo effect" that helped Apple nibble back market share in the computing space could reverse if its iPods give way to other media players or techno-gadgets.

Antidote? Who says the "halo effect" can't go both ways? The new "I'm a Mac, I'm a PC" campaign is brilliant, and one can argue that Apple's ability to work Windows-compatibility into its latest systems is a coup that will help Macs and Powerbooks become standalone winners. Besides, even if Apple suffers shipment delays, do you really think the iPod will fade as the standard media player of choice? Where else is a competitor catching on and branding tens of millions of players?

Loco like a locomotive
It's not just these three popular stocks that have to worry about missteps. As an investor, your job is to size up your portfolio and see where the weaknesses lie. It's more than just a healthy exercise to skewer your own stocks and vindicate your investments along the way. It's also a great way to spot declines early on, when you catch the company failing to steer clear of a head-on collision with its shortcoming.

That's sound advice whether you buy growth stocks or value stocks, small caps or blue chips. As part of the analyst team for the Motley Fool Rule Breakers newsletter service, I am perpetually picking apart our recommendations. Part of it is just garden-variety "game theory," in which you visualize how moves by a company, or even a competitor, would play out in the long run. Part of it is taking advantage of your objectivity to see pitfalls and opportunities before even the company itself becomes aware of what lies ahead.

It's what will make you a better investor. Embrace the kryptonite, but do so wearing kid gloves and safety goggles. and Netflix are Stock Advisor recommendations. Microsoft was singled out last year for Inside Value readers.

Longtime Fool contributor Rick Munarriz has been writing the "Early Adopter Roundup" column since the newsletter's inception in the fall of 2004. He does own shares in Netflix. The Fool has a disclosure policy. He is also part of the Rule Breakers newsletter research team, seeking out tomorrow's ultimate growth stocks a day early.