Microsoft's Standalone System Sweeper is a bootable antivirus tool that can help you remove otherwise undetectable malware, and perhaps get an infected PC starting normally again.
Don't be fooled by the small file size. This is just the downloader, and it'll need to go online to fetch the data necessary to build its boot environment. And this may take a while (there are more than 200MB of files involved), even if you've a fast internet connection.
Once the download is complete, though, you're able to create Microsoft's Standalone System Sweeper on a CD, DVD or USB flash drive. Or, if you know what you're doing, the setup tool can create a bootable ISO image and leave you to burn it wherever you like.
And after it's finished, just boot from your System Sweeper device, wait for the program to load, and click "Start full scan". Then just watch as System Sweeper scans your system for malware, which should be far more readily detectable now that you've booted from another disc, bypassing any stealthy protective measures that the virus may have employed.
It's no different to any other bootable antivirus disc. Every PC owner needs a couple to hand, though, just in case of emergencies, so if you're unprotected then go make your bootable Standalone System Sweeper disc immediately.