It's possible this was a takeover by the PDF Complete application. I haven't heard about these situations, and I recommend uninstalling PDF Complete immediately. Try using Revo Uninstaller. Even if it can't uninstall the application normally, it will force it to uninstall by wiping out all traces of its presence from the system. After this process, right click a PDF file and select Open with. Choose Adobe Reader (it's possible this is actually a file association takeover) and click OK. Now Adobe Reader is your default PDF reader.
