won't run .exe programs

I have up-to-date Panda virus software on my computer. After the last Windows updates download, the XP virus scanner windows kept coming up saying I had numerour infections and I couldn't get the windows to go away. I ran a full scan with my Panda which called for a reboot afterwards. The XP virus scanner windows didn't come back but now my desktop icons come up as "application not found" when clicked on. Using the start menu pop-up results in the same thing. Even spoke to Windows support online and downloaded their remote support files but they won't run, only a screen asking what application do I want to use to run these files. I got a "that's weird" comment from the online person (so encouraging, lol). A friend gave me a registry mechanic program on a USB plug-in stick but all that happens when I try to run it is the same window as before asking what application I want to use to run it. Dead end. Any help out there?