I'm currently running Kaspersky internet suite 2010, Check out the AV reviews, It has the highest detection rate.
As for Windows, It has to have all these patch's (Critical Update's) Because of sloppy programming. You dont hear of other OS's being updated this much, Because they are far more secure to start with.
AV-Comparatives - Independent Tests of Anti-Virus Software - Main-Tests
Read the Retrospect/Proactive report for May...
Both Kaspersky and MSE are in the Advanced+ category, both scored a 59%, they're essentially evenly matched, and guess what, MSE is free.
Ok, so with your logic, why are patches released for OSX? And why are there new kernel versions of Linux, or even updates for Linux released since they are infallible? Plus, why is OSX increasingly becoming LESS secure? You don't have very sound logic... M$ is more on the ball than Apple is for releasing updates. And no program is perfect, OS's included. They all need patches because of newly found/developed vulnerabilities.