Avira and Kapersky do rescue disks which are their scanners but run from CD. The Avira one can be found at
http://www.avira.com/...antivir-rescue-system and the Kapersky one is at
http://support.kasper...om/viruses/rescuedisk . Avira update theirs at least once a day, so there is no need to download any updates when it's running, and I imagine Kapersky do the same.
Whichever you go for, do the download on an uninfected machine, then burn to CD, boot the infected PC with the CD (you might need to change the boot sequence in your BIOS to do that) and run it.
Avira have two versions of their scanner for download, and ISO and and exe version. If you download the ISO version, you will need software which can burn it to a CD. If you download the exe one it includes some code to burn the CD, so double clicking it should burn it to CD for you.
Two things though. First there is no guarantee that either one will find whatever has infected your PC, and second there are some infections which will disable your ability to open, for example, Task Manager or it's equivalent in more recent Windows installations, even when you are logged on as the administrator. If that has happened, getting things reset is a bit of a pain, to put it mildly.