Useful links in: http://www.techmixer.com/free-bootable-antivirus-rescue-cds-download-list/

Steps:
1. Download the .iso-file
2. Burn the .iso-file to a CD. Can be done with IMGBURN (free) for example. Can also be done with Nero or (free) CDBURNERXP, but you need to make the right choice then (something like "burn image to disk")
3. Set the BIOS of the PC you want to scan to boot from the cd-drive
4. Put that CD you burned in the drive and boot from it.

Kees