Memory ! 3Gb is easy to overstretch, have 3.35gb available from 4gb vista 32bit and it hits 3gb threshold easy in crysis, 3.2 gb to be exact, but that is with 81 processes running, try killing any unused processes to free up some ram and see how it goes, have found Vista able to game on as few as 58 processes including Anti-Virus (Avast) still running!
Edit: Win 7 64 bit , not sure how stable crysis runs on this, maybe others have similar problem?