Summary: Microsoft has confirmed that its security tools erroneously removed the Google Chrome browser from Windows machines, marking it as a variant of the notorious Zeus (Zbot) malware family.
UPDATE: Microsoft has confirmed that this was caused by a faulty anti-virus definition update that affected about 3,000 Windows users.
Here's Microsoft's statement:
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.
A Microsoft spokesperson says affected users should manually update Microsoft Security Essentials (MSE) with the latest signatures.