A while ago I installed "Flight Gear" which is a free flight sim, but I got bored with it pretty fast.
Just yesterday though I found "YS Flight". It's the best free sim that I've seen. On the down side, the graphics are REALLY simple, and the sounds in it are pathetic, but the flight simulation is good and the combat is fantastic. Still not anywhere near AC sims (where can I get me one of those!?!?!)

http://homepage3.nifty.com/ysflight/frmindexe.html
(It's the first one in the list of downloads)
Note: since it's a home brew Sim, it's not super straight forward. You have to open a default flight or create one. Just follow on-screen instructions
Also, if you are willing to pay big bucks, pick yourself up the Cougar Hotas Joystick and throttle from Thrustmaster:
http://ftp.thrustmaster.com/pictures/PC ... /Hotas.jpg
All metal construction. Very heavy feel. Very realistic. I haven't tried them yet, but they look awesome! (anyone trying to get rid of a set for cheap???)

Anyway, try out the YS flight sim. It's fun and FREE. You can even fight over the internet!