Free you say?
Dabbsy - I think you've missed a point - I'm one of the Linux boys - yes I know, Linux isn't for you average Joe, having to occasionally drop into the command line, but it's getting better and easier to use.
There are two things that is worrying MS.
Firstly the obvious - it's free - forever, with as many upgrades as you like, with no tie-ins.
Secondly the security - no antivirus for me! This may, or may not be the norm in the future, but for now you've not got an annual tax a la antivirus. MS has had to abandon Tuesday patch day as there are too many zero day exploits turning up which need to be patched immediately. I my experience of Linux, an exploit rears it's ugly head and you get a patch the next day or so - sometimes the same day! and unles it's a major kernel update, you don't have to reboot every time.
The Linux desktop is coming and it's worrying MS and Apple. I know this, as for the first time a major software vendor, who which I cannot remember - damn my memory - stated that their package was available for download for Windows, Mac AND Linux. More and more software vendors are now catching on to Linux compatibility to expand their reach.
Linux is only going to get easier and more attractive to the public with major vendors now pre-installing their laptops with Linux, leading MS to offer Windows 8.1 very cheaply.
MS is worried - in 20 years time people will wonder why on earth people had to pay £100 or so for Windows 10 Professional.