You boot your troubled system using the Norton Bootable Recovery Toold CD/DVD , that allows your system to boot into a safe environment, you will/should have internet access in this environment too thus allowing you to update and run any scanners you so wish.
You do not need to download a new copy of NBRT every time, In that sense it is not like MS Standalone System Sweeper
This may be of interest to you, it gives you some info on what the NBRT is and does.
http://www.ilovefreesoftware.com/23/windows/security/norton-bootable-recovery-tool-create-system-restore-cd.html