It can happen sometimes that while running a program/application/particular software, Windows does not find the particular extension that is needed to execute the program. An error message will hence pop up to warn you that the file needed is either missing or has been corrupted.
What to do?
If the file that you are looking for is missing, you can try the following:
Go to the Control Panel and browse for the Folder Options
Once done, choose the desired extension (if it is one other than .exe) and choose the option Advanced
As soon as the option is opened select Open and Modify
Under the field labeled as Application Used make sure that it has the %1 symbols at the end of the extension. If not, you can add it yourself.
What if your computer is infected?
If your computer had been infected and that you cannot open your computer the file, you must absolutely install Ad-Aware SE Personal. Don't forget to always make an update of all your antispyware/viruses before making a virus scan.