When you try to launch applications [.exe files], an error message appears

This indicates that the .EXE file association in the registry is corrupt. This behavior is generally caused by viruses; one of which is SirCam virus, which modifies the .exe file association in registry. To launch applications , you may need to reset the .exe file association using any of these methods:

Click Start, Run and type Command
Type the following and then press Enter after typing each one:
cd\windows
regedit
Navigate to and select the following key:
HKEY_CLASSES_ROOT \ exefile \ shell \ open \ command

Double-click the (Default) value in the right pane
Delete the current value data, and then type:
"%1" %* [with quotes]

(ie., quote-percent-one-quote-space-percent-asterisk.)

Navigate to:
HKEY_CLASSES_ROOT\.exe

In the right-pane, set (default) to exefile
Exit the Registry Editor.

If you're unable to launch Regedit.exe even from Command Prompt, try this:

copy regedit.exe regedit.com
regedit.com