We are tracking down the recent launch crash that came with update 1021. Right now it looks related to DirectX 10 and compiling shaders and we are looking further into it for a hotfix or a rollback if it will require more work.

For a small portion of other users there has been some file corruption and the first few options here will tend to fix those:

http://support.tripwireinteractive.com/KB/View/55-killingfloor-crash--game-exe-has-stopped-working