aswMBR (also known as Avast Anti-Rootkit) is a free rootkit scanner designed to detect MBR/VBR/SRV rootkits (for example: ZAaccess, MBRroot/Sinowal, Whistler, Cidox, TDL4/3/Alureon, Pihar etc.)
The most powerful "feature" of this tool is the ability to write a new Master Boot Record. Most modern rootkits will alter/replace the existing Master Boot Record. "FixMBR" button will remove the suspected/infected Master Boot Record and write a new one. Most modern computers offers support for hardware virtualization and while using aswMBR, you will be prompted to use it (if your CPU has support for hardware virtualization) for a better rootkit detection. Moreover, there are other rootkit scanners such as TDSSKiller from Kaspersky Labs or Sophos Anti-Rootkit if there is no sign of an infection on your computer.
Important: Before attempting to scan your computer, you should always update this program by downloading the latest virus definition (you will be prompted when you launch the program - make sure that you choose - YES). Also, if you suspect a rootkit infection you should save the log and send/post it to a malware forum or to a security professional.