If your Norton software includes a firewall, it should alert you to attacks
from the outside and that only means your system is under attack, not that
anyone has gained access. Also note, if you are connecting through a
router, that along with a firewall would make if very difficult to access
your system.
Other than that, the only way to know for sure is if you see files missing
or changed. It's not all that easy for a user from the outside to gains
access because they would still need the proper permissions to access the
files and if you are using NTFS instead of FAT32, at the very least, they'd
have to know or have hacked your logon password.
If you have a broadband account and you are in the habit of leaving your
system unattended, you also could set the screen saver to force you to enter
a password in order to return to the desktop. Again, this helps prevent
someone hacking your data.
