Steam – Modern Warfare 3 Crashes on Launch: How to get it running

When I attempt to launch any part of Modern Warfare Three on my PC through Steam, it crashes immediately, and this happens (multiplayer as example):

Problem Event Name: APPCRASH
Application Name:   iw5mp.exe
Application Version:    0.0.0.0
Application Timestamp:  4eb37b9f
Fault Module Name:  StackHash_e8ad
Fault Module Version:   0.0.0.0
Fault Module Timestamp: 00000000
Exception Code: c0000005
Exception Offset:   003e0000
OS Version: 6.1.7601.2.1.0.256.1
Locale ID:  5129
Additional Information 1:   e8ad
Additional Information 2:   e8adce1c2b9e7be834b4063ac3c53863
Additional Information 3:   e8ad
Additional Information 4:   e8adce1c2b9e7be834b4063ac3c53863

When I run the exe's directly from the game directory, the splash screen appears and says that Steam must be running, even when it is.

Is there anything I can do to get this running?

Thanks!