Microsoft released a special Malicious Software Removal Tool update on Tuesday to better eradicate the Afcore botnet.  This is described below:
MSRT – Second release during April 2011 to fight Afcore botnet

QUOTE: The 4th Tuesday of the month is also a Patch Tuesday, though lesser-known. Microsoft issues non-security updates on almost every 4th Tuesday of the month, but this month has a surprise addition: An extra edition of the Malicious Software Removal Tool.  The updates released today fix a variety of problems and Microsoft is, as typical, vague about them. One addresses application compatibility problems; two are “reliability updates”; and two more “resolve issues” in Windows.

The last update is the MSRT. Microsoft always releases an MSRT on the first Patch Tuesday of the month, but this may be the first time they have released a second. This version is part of an ongoing effort to take down the Afcore botnet. Win32/Afcore’s authors released new variants at about the time of the last MSRT release 2 weeks ago. The new MSRT also includes updates for other malware families.