Boot and clean a virus infected PC, with MS Standalone System Sweeper

Microsoft Standalone System Sweeper is a free recovery tool created by Microsoft, that will help you start an infected PC and perform an offline scan to identify and remove rootkits and other malware.

It can also be used if you cannot install or start an antivirus solution on your PC, or if the installed solution is unable to detect or remove malware on your PC.

The program is not a replacement for a full antivirus solution; it is meant only to be used in situations where you cannot start your PC due to a virus or other malware infection.

Check also this post for better security measures.