NT systems are computers running a OS (Operating System) based on NT technology. The OS's are XP, 2K, NT and 2k3.

The special thing about these NT-based systems is that they can schedule programs or tasks to load when the system reboots (something like that.)
If you have any of the systems mentioned by Eddy you can schedule avast! to run a boot scan.

If you have a NT based system, run a boot scan.
And spend some time reading the help to make yourself familiair with the options Avast is offering to the user.
If you really want to become familiar with avast!, reading the help files is one of [b]THE[/b] most important things.