Oh boy! Apparently the Microsoft security products didn't like that upstart web browser and decided to remove it , considering it to be a 'Trojan'...
"On September 30th, 2011, an incorrect detection for PWS:Win32/Zbot was identified and as a result, Google Chrome was inadvertently blocked and in some cases removed from customers PCs. We have already fixed the issue - we released an updated signature (1.113.672.0) at 9:57 am PDT - but approximately 3,000 customers were impacted. Affected customers should manually update Microsoft Security Essentials (MSE) with the latest signatures. To do this, simply launch MSE, go to the update tab and click the Update button, and then reinstall Google Chrome. We apologize for the inconvenience this may have caused our customers."
(from ZD Net)
(also more info here!)