My network seems to be particularly vulnerable to bogus security malware - even though antivirus/antimalware is in place.  What is odd is that quite often the problem seems to crop up right after a windows update has been automatically installed.  Has anyone else noticed this or is it just a coincidence?

Likely it's a coincidence because windows updates are SUPPOSED to reduce known vulnerabilities.

I'd look toward your users and see what they're doing on the internet or if they're using memory sticks from home.

Also, ideally, you would have a layered approach in place to protect you, such as:

1.  Good firewall for the entire network
2.  Spam/malware email filtering function whether inside your network or farmed out to a someone that filters your mail and passes it on.
3.  Web site monitoring/blocking function
4.  Security software on individual PCs.

But if the workstations are unblocked in terms of web surfing or plugging in flash drives, users can thwart all protection upstream of the workstation.