Once a virtual monopoly, Microsoft's (NASDAQ:MSFT) Windows business is but a shell of its former self. Apple's (NASDAQ:AAPL) iOS and Google's (NASDAQ:GOOGL) Android have exploded in popularity in recent years, and the demand for mobile devices has weighed on the market for traditional PCs.

But 2014 could be the beginning of the end for Microsoft's Windows. More than just mobile devices, there are growing signs that Microsoft is losing control of the traditional PC. Let's look at five of those signs.

1. New Android-powered desktops
Last week at the Consumer Electronics Show, the world's top two PC vendors, Hewlett-Packard and Lenovo, unveiled radical new desktop PCs. I call them radical because, unlike the vast majority of desktop PCs, they run Google's Android -- not Microsoft's Windows.

HP's 21-inch all-in-one Android desktop is aimed at businesses, while Lenovo's machine is intended for home use. At any rate, if Android-powered PCs catch on, Microsoft's Windows will suffer.

2. The growth of Chrome OS
Google's other operating system, the Web-dependent Chrome OS, has been around for years but has finally started to gain traction. Amazon.com said two of the top three best-selling laptops during this past holiday season were Chromebooks, while Google's operating system now powers about one out of every five laptops sold through commercial channels.

Last week, Toshiba unveiled its first Chromebook, joining a long and growing list of Chromebook vendors. Almost every PC-maker will have at least one Chromebook model on sale in 2014.

3. Larger, enterprise-focused tablets
Also on sale in 2014 will be larger tablets aimed at business users. Samsung's 12.2-inch Galaxy Note Pro is a tablet powered by Google's Android intended to appeal to enterprise users. With a screen almost as large as those of many laptops, and a plethora of built-in business software, Samsung's device should cannibalize sales of Windows-powered PCs.

Apple is expected to join Samsung later this year. A number of reports have indicated that Apple is working on a 12- or 13-inch iPad "Pro" that will go on sale next fall. According to analysts at Evercore Partners, the iPad Pro will be pitched as hybrid between a tablet and a traditional laptop and will be aimed at business users.

4. Steam Machines
Outside the office, Microsoft's Windows still has a following among gamers, but they, too, could soon abandon their Windows PCs in favor of ones running an alternative operating system. Valve's Steam OS is based on Linux and aimed at gamers. A dozen different PCs running Steam OS -- "Steam Machines" -- were on display at CES last week and will go on sale later this year.

Steam OS can't do all the things Windows can, but if gaming is what you do most on your PC, Steam OS could be a potent alternative to Microsoft's operating system -- especially because it's free.

5. Cheaper Macs
Then there are Apple's Macs -- obviously, Apple has been a force in the PC market for decades, but its Macs have never taken more than a token share of the overall market. I wouldn't expect that to change dramatically, but Macs could become increasingly popular machines in 2014 and beyond. Historically, the problem with Mac adoption has been one of price: The absolute cheapest Mac models start at $999; the average PC sells for about half that amount. Put simply, most people just can't afford them.

But Apple has started to cut Mac prices. When Apple refreshed its Mac lineup in October, it cut prices of its MacBook Pro models by 13%. It also announced that its new operating system will be free to all users, and that buyers of new Macs will get several Apple-made applications for free. With Mac prices on the decline, Apple could take a larger percentage of the PC market. There are already signs this is beginning to take place -- according to Gartner, Apple's share of the U.S. PC market rose to 13.7% in the fourth quarter, up from less than 10% last year.

Is Microsoft rushing Windows 9?
According to longtime Microsoft observer Paul Thurrott, Microsoft is working hard to get Windows 8's successor -- Windows 9 -- shipped by April 2015. Thurrott notes that Microsoft is well aware of its Windows-related problems and is working hard to rectify the situation.

But in the tech world, 15 months can be an eternity. With Microsoft's Windows under siege from seemingly every angle, by the time Windows 9 is released, it could be too late.