The Microsoft forefront anti-virus software has been incorrectly identifying Chrome as a PWS:Win32:Zbot infected executable.  Microsoft have confirmed the problem

"On September 30th, 2011, an incorrect detection for PWS:Win32/Zbot was identified. On September 30th, 2011, Microsoft released an update that addresses the issue. Signature versions 1.113.672.0 and higher include this update."

The problem was first observed around 17:00 this afternoon and the new corrected signature has been deployed at CERN at 21:45 and verified.  Systems which do not automatically retrieve the signatures should refresh their settings from the Forefront “Help” menu.

If users have found their chrome.exe removed, a new version can be downloaded and installed.  Settings and bookmarks are preserved.

Look at the Knowledge Base Article: KB0000900 which explains step by step how to restore Chrome.