Microsoft Watch's Joe Wilcox, in solemn observance of Friday the 13th, compiled a list of Microsoft's ten most unlucky breaks. The strokes of misfortune chosen were weighted according to heinousness (with #10 being least signficant, and #1 the most).
Checking in at #7 is the development of the Linux kernel. Putting aside Microsoft's whole Schrodinger's cat sort of approach to Linux over the years (Linux is not a threat. Linux is a threat), it is interesting Wilcox (and the analysts who helped him narrow down the list) focused on the kernel as the bad break.
I suppose the reasoning is that the Linux kernel powers alternative operating systems, and a large part of Microsoft's business focuses on its own operating system, ergo the development of the Linux kernel was where the bad luck originated. There's probably some truth in that idea, but it's by no means the whole story.
What's interesting is Wilcox touches on the circumstances that led the Linux kernel to become what it is in a rather passing manner. Linus Torvalds developed the Linux kernel in 1991, and then licensed it under the GPL. The concept and spirit of the license were spawned in the mid-80s, when Richard M. Stallman left MIT to pursue the GNU Project and ultimately form the Free Software Foundation. Without an open license of some variety, would the Linux kernel have gotten the foothold it did? Would it have seen the light of day? And more relevant to the "unlucky" discussion -- would open source software be an unlucky break for Microsoft if the Linux kernel had never been created?
My guess is yes, it would still be a thorn in Microsoft's side. If Linux didn't exist, if the open and free BSD distributions weren't available, I believe Microsoft would still be troubled by open source applications. There's enough evidence of this in recent history. The debate goes on whether Firefox and Chrome may rekindle the browser wars, but Internet Explorer isn't oft mentioned as a contender in that arena. It's an interesting point -- did it lose the previous war with Netscape? Netscape is gone, and though IE remains, it's accompanied by a new cast of open source competitors. There are undoubtedly areas in the Windows world where open source has a good, strong hold.
Open source simply isn't operating system dependent. Microsoft is in the operating system business, but Windows isn't its only product -- the availability of open source alternatives to the other software products it sells would remain bothersome (maybe even unlucky), even if the Linux kernel had never been created.