While trying to sign an EXE file with my code signing certificate SignTool kept failing with this error:
SignTool Error: SignedCode::Sign returned error: 0x800700C1
Either the file being signed or one of the DLL specified by /j switch is not a valid Win32 application.
The error was a result of the EXE being already signed.
Someone wrote a tool called delcert.exe which can easily remove existing code signatures. The tool is available for download from the XDA Developers Forum