If your system becomes badly compromised by malware then one approach to fixing it is to boot from a read-only disk (such as a CD disc) and run a scanner from a safe, uninfected environment.
Kaspersky provides a free bootable rescue CD that includes an anti-virus scanner. Other vendors do the same, but I mention Kaspersky here because a previous article, about an older version, is one of the most popular posts on this blog and still attracts large numbers of visitors. Hence the belated update.
The disc contains a live Gentoo Linux operating system that can run in text or graphical mode. It is compatible with 32- and 64-bit x86 systems (i.e. normal PCs). As you might hope, you can download updates before running a scan.
Interestingly, if you are scanning a system that already has a Kaspersky anti-virus program installed the updater will check the update files already available and only download those that are not available locally.
A utility called Kaspersky USB Rescue Disk Maker is also available from the site. It takes the contents of the Rescue Disk and copies it to a USB device, which you can then use to boot an infected PC.
More documentation is available from Kaspersky's Technical Support site.