notepad.exe is a mian componment for windows. It needs it. lol. On a rare insance this happen's my first idea would be click the start button, or hold the windows key, and the task bar should open a nice bar, click RUN, and type this notepad.exe. This should open it. You need no directory etc. If that in turn does not work, then try this open Find. Select your main directory, C: tc, then type notepad.exe and search it. This should pick it up. If not inform me.
For norton, yes uninstalling an older build is necessary. Yes some files are most likly left behind. You can go through and delete them, but id advise you not to. Only because they may have embedded in to another program, and migh still be needed by the system. Unless your sure then do not try anything you dont know fully understand.
it is now safe to install. If there are any more issues please by all means Post etc.