can't open excel files except from excel itself

I can't open excel files from my computer or excel attachments in emails. I can open files if I first open excel and then find the file and open it that way, but I can't just click on an excel attachment-- excel just doesn't open. Same if I try to open an excel file directly from "my documents". I am using Excel '07 and this problem just started today-- this was not happening for the prior several months. Please help. Do I maybe need to reinstall excel or something like that?