Microsoft have been working on a new standalone antivirus, named Microsoft Standalone System Sweeper (MSSS?), which scans for viruses and spyware without the need to boot into Windows. This is currently in beta, but seems to work very well.
If you’ve used Microsoft Security Essentials, then you’ll already be familiar with the interface as it’s very similar and even uses the same virus and spyware definition files. To make use of System Sweeper all you need to do is download the 32bit or 64bit version (depending on your version of Windows) and install it to a USB flash drive. At the time of writing this post it takes up around 260MB of space, so a flash drive of around 512MB should be fine. Once you’ve installed MSSS simply reboot your computer, choose to boot from your flash drive and then scan for viruses. If any are found simply quarantine or remove them.
Microsoft Standalone System Sweeper isn’t a replacement for an antivirus, and is really there to help remove viruses that stop you either running your own antivirus scans or prevent you from booting into Windows altogether. If you need a decent but free antivirus, then consider Microsoft Security Essentials. It’s completely free and I recommend it to all my own customers.