Can't install any programs on computer

Hello,

Please help. I cannot download/install anything from mozilla firefox and my internet explorer doesn't work at all. When i try to download things, like games or other programs, it either won't start the download or i think it downloads but won't install. It will come up in my downloads window but when i click on it to open in install it says application not found. it also does that when trying to open somethings on my desktop, like microsoft word. I've switched from trendo-micro to norton security so idk if that did anything. I'd really appreciate if someone could help me. Thanks.