The USB drive boots up as Linux. It then scans your Windows drives for malware (using Kaspersky, Sophos, and Vipre). Doing so it is completely immune from any Windows malware on your computer since it is doing the scanning from Linux, not Windows. Windows is not running at all when PCLife operates. PCLife runs exclusively under Linux when it checks your Windows drives.
Most computers made in the past several years allow booting from USB, so select that during boot time or in the BIOS. If it won't allow that, then start Windows and click on the PCLife icon as the USB drive and it will install a temporary boot record / boot manager to let the computer boot the USB drive. Either way works.
The license lasts for one year and you can scan up to 3 computers any number of times within a given month. In the next month, you can scan another 3 computers. When the license runs out after 1 year, it needs to be be renewed on the PCLife web site for another year.