The MicroFlaccid Clown Show that Is Vista

Yep, their update, Vista SP 1, breaks antivirus and firewall programs, in addition to putting some machines into an endless reboot loop.

A major update to Microsoft's Windows Vista operating system could leave computers vulnerable to hackers and malware as the service pack prevents several widely used antivirus programs from operating, the company said.

The list of security products that Windows Vista Service Pack 1 blocks includes Zone Alarm Security Suite 7.1, Trend Micro Internet Security 2008, and BitDefender 10. It also blocks the 2008 version of the Jiangmin antivirus product.

I don't know of anyone who is using Vista who does not regret it.