There's a rational explanation. Sure, we dream often of what we do much of the day, however, when we sit at the PC, we don't focus on the concept of the PC but rather what we're doing at it (watching a movie - on the screen, reading a forum - on the screen, etc.).

I've had dreams where I was browsing forums or even chatting in IRC for a small amount of time but the PC was never really present. I was just witnessing what was going on on the screen.