With the GeCad and now the Sybari purchases by Microsoft, it is widely speculated that they will make some sort of announcement at the RSA Security Conference during Bill Gates' keynote address regarding a desktop and / or enterprise level AV solution.

Everyone seems to have their favorite AV program- Trend, McAfee, AVG, Sophos, Symantec, etc. I am curious, what features or components do you feel are necessary to make it a good AV product? Or, why do you prefer one product over another?

Aside from aesthetics and support and such, getting down to the nitty gritty- what does the program need to do on a technical level in order to effectively protect against malware- specifically on a Windows platform?