What was the first game on PC that you put a lot of time into? It can be an online game or offline game. It can also be as old as you want it to be. I think the first game I really put a lot of time into was sort of this counter-strike ripoff called "CrossFire". Back then I had a pretty poor PC which couldn't run a lot of things. It was a fun game for what it was, and I put hundreds hours of money and time into it. It's sad that it didn't really go anywhere, but it helps me out in other FPS games I play on the PC now, having some experience.
If so, Earthworm Jim is one of the very first games that I discovered as a kid. I was using a SEGA emulator and this game was taking all my time because it was so much fun. If that doesn't count then the very first 'official' PC game that I played would be Deus Ex. I bought the game on a whim because it was 2 games for about $3 and I saw guns on the cover and said, hey shooting people in 3D can be fun. The game that came bundled with it was Thief 2 but it was very hard for me to get into that kind of game at that age so I stored it away and still have it actually, working. I had dial-up at the time so downloading big games was out of the question. I bought more $1 CDs from a classmate (he was like a videogames dealer at school lol) that included a pirated GTA San Andreas and a pirated CS 1.6. My PC was a Pentium 4 with 128MB of video memory so I had no way of playing anything bigger than that until much later.