try to open ur steam and go to ur games library click on ur game right click and properties then make automatically upfate that game.....restart steam run the game again it should install the updates auto before u enter ur game...
Thanks for the reply, but I think I solved the issue on my own. For anyone who runs into this problem all I did was re-install my video card drivers and physx drivers (using the "Driver Sweeper" software to wipe them clean from my PC first) then it ran just fine. Still had to turn off PhysX support in the game options because of some notorious slowdown in level 3, but its no biggie.
Also, I'm not sure if this is true, but I've also read that the Mirror's Edge patch is already installed in the Steam version.