If Microsoft's (NASDAQ:MSFT) next operating system, Windows 10, catches on, it could pose an immense challenge to Google's (NASDAQ:GOOG) (NASDAQ:GOOGL) search business. In many ways, Microsoft is adopting Google's Android playbook with Windows 10, giving away the operating system for free in an attempt to spur use of its online services.

Windows 10 is jam packed with features that tie directly into these services, in particular, its search engine Bing. For years, Google's virtual desktop search monopoly has been unchallenged, but strong Windows 10 adoption could take a toll on Google's share of the search market.

Cortana searches right from the desktop
To be fair, both Windows 7 and Windows 8 include a level of integration with Microsoft's Bing, but they are relatively modest and to some extent, impractical. Windows 7 users, for example, must go through the trouble of installing "Bing Desktop"; Windows 8 users can see Bing results directly from Windows' universal search, but it's an ugly, jarring process that forces one into Microsoft's modern interface.

Windows 10, in contrast, has a search box built directly into the task bar. It's powered by Microsoft's digital personal assistant, Cortana, and roughly resembles the Google search bar that's typically present on the home screen of most Android phones. Typing into the bar brings up a small, unobtrusive window that allows a user to search through local files or on the web.

It even offers hands-free support: Saying "Hey, Cortana" allows a Windows 10-powered PC to search the web via dictation. The feature is identical to the "OK, Google" command built into modern Android phones, but obviously relies on Microsoft's Bing rather than Google's search engine.

Of course, Windows 10 users won't be forced to use the search features if they don't want to -- they can easily open up a browser window and search from there -- but many may find them compelling.

Spartan could win over some Chrome fans
If they choose to search from a browser, it may be Microsoft's Project Spartan rather than Google's Chrome. Microsoft's new, more modern browser, is set to ship with Windows 10. Unfortunately, we still don't know much about it. Although Microsoft confirmed its existence, and demonstrated a few of its features, the Windows 10 preview does not (yet) include it. But what we do know about Project Spartan is promising.

Microsoft's new browser, for example, allows users to annotate web pages, and includes a built-in reading mode. It also, most notably, features Cortana, which (like the Cortana-powered Windows 10 search bar) should entice Project Spartan users to search with Bing.

OneDrive keeps getting better
Lastly, Microsoft's online cloud storage service, OneDrive, enjoys a few massive improvements in Windows 10. OneDrive is already heavily integrated into Windows 8 and Microsoft Office, but Windows 10 takes it to the next level entirely, with new features like remote file access and better backup and sharing options.

Cortana also integrates with OneDrive, allowing users to quickly search through their cloud files. Windows 10 users, then, that may be using Cortana for desktop search and in Project Spartan, will be encouraged to keep their files on OneDrive rather than competing services, such as Google Drive. That works in reverse, too, with OneDrive users more likely to search with Cortana (and therefore Bing) instead of Google.

The benefit of offering a free OS
Microsoft will give Windows 7 and Windows 8 users Windows 10 for free. That could take a toll on Microsoft's Windows-related revenue, but Windows 10's deep integration with Microsoft's online services could more than make up for it in the long-run.

Windows 10 users won't be forced into Microsoft's online ecosystem, but the deep level of integration could make it an enticing prospect. Unfortunately for Google, the use of Microsoft's cloud-based services -- Cortana, Bing, OneDrive -- could come at the expense of its own -- Google Now, Google Drive, and most notably, Google Search.

To some extent, this may not matter -- Microsoft is not a strong factor in the increasingly dominant mobile space, and Windows appears to be in decline. Google's Android remains immensely popular, and Google's Chromebooks are attracting interest.

Still, strong Windows 10 adoption should be seen as a negative for Google and its shareholders.