Getting error message, "Windows won't allow this program t be opened", when I click on "Control Panel"(DEP)?

Downloaded all this Verizon Yahoo Online Protection stuff (Norton Anti-Virus) Now I can't open "Internet Exporer", or open my "Control Panel", amongst other stuff. I'm running Windows XP on an older Dell computer w/ pentium III. The actual message reads, "Data Execution Prevention'-Microsoft Windows. " To Help protect your computer. Windows has closed this program." I can't open," My Computer", "Search", "internet explorer", etc...Anyone have a clue, as to how I can re-open these programs?