PC gaming doesn't have clearly defined "generations", but something much more fluid that every once and awhile shifts so much that a new epoch can be defined to start a new era in PC gaming. Think of it like constantly moving tectonic plates and the eventual earthquake that happens when enough things move at the same time such that the hobby breaks off from the past and starts moving forward.
The problem is that it's often hard to see the big shifts happen until well after they've passed.
For example, I like to take 1998-2000 or so and define it as a "era" of PC gaming. It was defined in hardware by the rapid maturation of 3D video cards beyond the 3dfx Voodoo/Voodoo2. It was defined in software by a certain purity and refinement of genre gaming-shooters were pure in form and a lot of stellar multiplayer and single-player games-from Unreal in summer '98 to No One Lives Forever in 2000. RPGs made a strong comeback thanks to Bioware's efforts, and EQ broke open the MMO genre-but they were very "pure" RPGs. Console porting was rare, due to the limitations of the consoles at hand (PSX, N64), and publishers were still funding a lot of the more niche PC genres-turn based strategy games and space sims, for example-with decent budgets. There wasn't a lot of cross-pollination between genres, though, and publishers hadn't quite tightened their belts, either. Games not created in the US or UK didn't see much play here.
Something kinda happened in 2000-2001, and I can't figure out what precisely that was, but I'm peering at the consoles and the dot.bomb economy and I'm sure they had some part in it. Publisher money for midrange titles dried up, and the era of the multiplatform blockbuster (now that the new consoles were out) was really at hand. Overall game diversity has declined, and game company fragmentation/publisher-fucking is probably to blame. Publishers are less willing to spend money on traditional genre titles, and indie developers are making big strides in filling the niche. Hardware continues its march and, for the most part, continues to outpace consoles by a good shake. I'd say that the era is best started from May 2000 when Looking Glass went under, even though some notably previous-era games came out after that, it kind of represents the end of the strict developer-publisher PC gaming model that had been in place for a long time.
I'd say we are rapidly approaching the end of this era as well-and moving to a new terrain where traditional genre games are produced almost exclusively by overseas developers or indie shops, and the big-budget games come exclusively from developers wholly-owned by their publishers. Online gaming is completely mainstream, and any game with a budget even close to "significant" either goes to a console or comes from one. Hardware progress continues unabated, but the platform scales to such a degree that frequent upgrading isn't nearly as necessary at it once was.
