Norton releases updates usually once or twice a week, as long as you keep it up to date it will usually stop almost anything, however, new viruses spread fast and arent always caught.

Keep in mind you're risking a virus every time you download an executable (EXE, BAT, SCR, PIF).

Windows XP keeps a backup copy of most of the system files in a hidden folder and generally isn't quite as vulnerable, but it's not exacly exempt either.