I am looking for some simple multiplayer games like Gauntlet SS or Xmen Legends to play with my Girlfriend. are there any other decent coop games out there that are not overly complicated or difficult?

Star Wars Lego was fun for my daughter and I, though the Pod Race was overly difficult in comparison with the rest of the game.

is star wars lego coop. how long does it take to beat it. I havent really looked into star wars lego but I will do that now

Sounds like you're talking about console games. The Baldur's Gate games are pretty simple and fun for co-op.

Yeah, it is coop. There might be a few parts that isn't, I don't remember. It wasn't too terribly long but there are lots of unlockables.

Halo 2

I would say Fuzion Frenzy and/or something from the Norrath series.

What system(s) do you have?

right now I have an XBOX and a 360, but my roomate has the PS2 and GC so I can play those also

the adventures of cookies and cream on the ps2 is a favorite in my home.
I perfered dark legecy over secret sorrows of the gauntlet series myself.

You may want to take a look at the Hunter series. Both of the games on Xbox can be found rather cheaply and they were a blast multiplayer.