The family added to the February release of the Malicious Software Removal Tool is Win32/Sirefef. Win32/Sirefef is a highly prevalent complex multi-component family which continues to evolve. The payload for current variants may include such actions as modifying browser search engine results, generating pay-per-click revenue and performing Bitcoin mining on an affected computer.
http://blogs.technet.com/b/mmpc/archive/2013/02/12/msrt-february-2013-sirefef.aspx
