Firstly, any files on your hard drive will be deletable. Norton usually puts them in Program Files under both Norton and Symantec. You then need to check C:/Document and Settings/User(your main user but check all)/Local Settings and Application Data. These folder may be hidden so click on Tools - Folder Options - View tab and Show hidden files and folders.
Personally, I always clean registry entries but that is really up to you. Norton is a pain to clean. Some programs will probably remove the entries for you (not sure which as I do it manually, although Eusing Registry Cleaner is free).
Click on Start - Run and type in Regedit. Then do a search for both Norton and Symantec and delete the registry keys associated with them. If you are not familiar with registry editing, then I strongly suggest that you a) back your registry up first and b) move the keys to a folder rather than deleting them outright. Usual disclaimer applies.