According to some files located inside of the latest ChromeOS, Google is readying builds of its OS for three different hardware makers.  HP (with reservation), Dell, and Acer.

If those names sound familiar, it is because they are the world’s three leading PC manufacturers according to IDC and Gartner data. That market is a monster and growing (27% according to today’s numbers) and until now, Microsoft (MSFT) has owned the OS on 90+% of them.

In fact, by targeting these three companies, Google (GOOG)  lays claim to more than half of global PC marketshare. That’s not a niche market Google is looking at. That is going after the jugular. Ubiquity.

Should Microsoft be worried?

Well, along with Google’s free Apps attack on Redmond’s other cash cow, Office, Google is now offering a free alternative to Windows.  It certainly isn’t good news for Microsoft that their three biggest customers have files in Google’s free ChromeOS.

However, this battle is long from over (or officially beginning, really).  Sure there are significant problems with Windows and it is expensive to maintain.  But, Google will have to convince the broad swath of PC users that they can operate only from the browser (with some help from Chromoting).  Can people trust the Cloud with their photos and documents?  Is their Internet connection reliable enough to bet their computing experience on?

Only time will tell.

Google’s ChromeOS is set to ship this Fall.