#1 - Microsoft

It might come as a surprise that the number one culprit in the death of PC gaming (as it was known) would be a company vital to the PC industry as a whole, but hear us out.  The Xbox platform (listed earlier) has provided a refuge for PC developers on consoles and is a Microsoft product.  The most recent operating system (the backbone of Microsoft's business) Windows Vista (also listed earlier) seemed to hamper the growth of PC gaming. But it goes deeper than that, though, with their publishing wing Microsoft Game Studios putting greater emphasis on console game releases (like Lost Odyssey, Viva Pinata and Gears of War) than on PC gaming releases.  The company has also purchased PC oriented developers like Bungie (which is now independent again) and Lionhead and pushed their franchises onto the Xbox platform.  What was perhaps the cruelest cut of all, however, was the closure of ACES (Flight Sim) and Ensemble (Age of Empires, Halo Wars), two long-time successful PC developers that didn't happen to fit with Microsoft's future business plans.  All we can say is congratulations Microsoft: you've killed PC gaming.