I disagree. Men on the ground with Ak-47's or shoulder launched weapons are the single biggest threat to helicopters in this day and age. The reason is that they cannot be seen. A n Apache flying NOE and/or performing observation or launch are very close to the ground and within easy range of small arms fire. It will be only happenstance that they encounter small arms but when there are thousands of troops on the ground it will happen.
The ZSU's are radar guided and easily spotted. Even without the ZSU radar being active the vehicles themselves are passively detected by Longbow radars and until identified can be approached with caution. (One of the failings of a radar like Longbow, or most any radar, of course is the ability to reliably distinguish between a ZSU and a Toyota pick-up)
Actually, one of the big advantages of the Longbow Radar was it's ability to use its millimeter wave resolution and sophisticated digital signal processing algorithims to classify targets as tracked or wheeled, and in some cases even friendly vs. hostile based on the radar signature. The system priortized detected targets as to friend/foe and threat level. Now, having said that, it did have its problems with reliability and making correct decisions.
Also, the Longbow Apache (AH-64D) is capable of firing a mix of both Radar guided hellfires (the RF Hellfire) and/or the original laser guided SAL (semi active laser) Helfire.
There is a lot of other stuff happening in the Longbow Radar processer that we can't talk about here, and Hack probably doesn't know about, either.
(I was a member of the test team for the Operational Test of the AH-64D in Jan 1995 at Fort Hunter Liggett.)