Atomic Bomberman is our all-time favorite family game on HTPC. Yes, it's old, but it runs perfectly on XP and it's downright mayhem from start to finish - every time we play. Up to 10 players simultaneous, if you get that many controllers going. We play 5-6 players, and we have yet to come across a PC game more fun than this. Any time we ask my wife to play, she's in, even though she is not a gamer. Our 5yo daughter can compete "enough" to have a good time, which is a big plus. We started playing this networked many years ago, and it transitioned flawlessly to the living room. There is a level of "cartoon" violence, along the the lines of Bugs Bunny and Tom & Jerry.
Trackmania United has fast-paced "hot-seat" arcade racing that our boys (6yo-13yo-15yo) and I enjoy a good bit. Once in a while, we'll even get my wife to jump in. Our 5yo daughter does not compete (obviously), but I give her a year or so to start kicking my rear in it. Yes, our 13- and 15-year-olds usually dominate, but it's still fun for all of us. Because we play a timed knock-out mode, the best players get the least play time -- perfect for our broad age range.
Micro Machines v4 is another arcade racing title, up to 4 players simultaneous. We have tons of fun with that -- arcade racing with weapons and powerups.