Some viruses corrupt the registry and force all applications to open the virus instead. This problem is usually resolved with a complete antivirus scan, that is if you can get it to scan. On a normal PC, there’s a registry key under HKEY_CLASSES_ROOT that specifies what happens when you double-click on an executable file .exe – but on a virus-infected system, this value is rewritten with the virus executable. That’s how it prevents you from opening anything.
To fix the problem, download, extract, copy this .reg file to the infected PC, and double-click it to add the information into the registry.