lost exe file association
This happens typically due to a recent virus infection: when you click on a shortcut to run a program, like MS Word or Internet Explorer, the computer does not know what program to use in order to open it. Download and run the following registry file to fix that.
For Windows XP - download the XP registry patch
For Windows Visat - download the Vista registry patch
If your system won't allow you to run REGEDIT or to even open .REG files, press CTRL-ALT-DEL and open Task Manager. Once there, click File, then hold down the CTRL key and click New Task (Run). This will open a Command Prompt window. Enter REGEDIT.EXE and press Enter.
FYI: regedit.exe lives in the Windows folder of the C: drive.
Refer to the following website if you have lost file associations for files other than EXE file types: