The thousands of pieces of information thrown at investors can be daunting. Analyzing balance sheets, comparable financial measures like earnings multiples and cash flows, and assessing management can make studying stocks a laborious task. However, lost in the information overload is that a buy or sell decision on a particular company can often be defined by a single question or theme.

With that in mind, in this series we're looking at the one key theme to look at for some of the most popular stocks around. Today we're digging deeper into Microsoft (Nasdaq: MSFT).

The one thing to know about Microsoft: How long can the Office and Windows gravy train last?
A lot of media attention paid to Microsoft focuses on the company's Xbox and online ambitions. While those are both key initiatives for the company that take up quite a bit of management focus, in the end, the quality of Microsoft as an investment comes down to its Office and Windows product lines.

That's because even after some adjustments for general overhead losses, Windows and Office contribute 92% of the company's operating profits. The thought that Windows and Office are important to Microsoft might elicit a "duh" response, but the outsized way in which they define Microsoft's future isn't always appreciated.

To put that in perspective, let's look at virtualization. That's a market Microsoft has pushed hard into recently, and it should:  Virtualization is the key technology that's fueling cloud computing and vast changes in the data center. Its key rivals in the segment, VMware (NYSE: VMW) and Citrix (Nasdaq: CTXS), are worth $37 billion and $13 billion, but those valuations are built on annual earnings of just $294 million for VMware and $271 million for Citrix.

Compare those figures with Microsoft's $20.6 billion in earnings.

If Microsoft started gaining gobs of market share in this segment under a best-case scenario, would its market capitalization rise by tens of billions? Probably not: The minimal bump to earnings would still be overshadowed by larger concerns to the company, like slowing PC sales and threats to its Office line.

Keys to the takeaway
I don't mean to infer Microsoft is unwise to pursue areas like virtualization. The key takeaway here is that the investor who can accurately predict how well sales of Microsoft's Windows and Office hold up in the coming years will be better at predicting Microsoft's share price than one focusing on virtualization, gaming, or its online ambitions. Not only do Office and Windows contribute a considerable amount of profit, but the clouds around their future contribute to the discount seen in Microsoft's share price.

For all the litany of products Microsoft makes, being able to predict how well PC sales hold on in the coming years against Android and iOS-powered tablets and determining how long businesses keep shelling out for Office will measure almost the entire worth of Microsoft. If you feel like you've got a good handle on that prediction, 90% of the legwork of investing in Microsoft is complete.

Eric Bleeker doesn't own shares of companies listed above. You can follow his tech stories and musings on Twitter. Microsoft is a Motley Fool Inside Value recommendation. VMware is a Motley Fool Rule Breakers selection. Motley Fool Options has recommended a diagonal call position on Microsoft. The Fool owns shares of Microsoft. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.