The steps I'm giving, are in no particular order, and I can't garentee for absolute accuracy, this is the method I have found to work, when removing modules in general:
- deleted symantec (in this special case) module folder
- in DB
- prefix_module table, delete symantec entry
(when deleting other modules, delete module related tables)
- under template_c delete db%3Asymantecalert_block_alertblock.html
(or any templates related to the module removing)

Though Note: some modules make actuall changes to the preexisting installation. This information should be in a installation or readme file.

Hope this helps.

I've had that problem, when I hard coded URL's into a theme. I then realized, that I had a missmatch in virtual URL's.

Hope this will help, and put you on track