Normaly when this problem occurs, a quick re-install does the trick. However, I believe you have tried this?

Symantec have a few tools on their web site that help with the removal of there software, I can't link you directly to them, as I don't know the version you are using?

The other method of getting 'Corps Addition' of your box, involves a reg hack that takes ages. This involves deleting all the 'Symantec' and 'Norton' entries in the regestry, with the exeption of those that come when you install Windows. It's a **** load of work.