While publishing a ClickOnce application using Visual Studio 2010, it may fail with the following error message:

Error 1 Cannot publish because a project failed to build.   
Error 2 An error occurred while signing: SignTool.exe not found.

Build output:

Building WindowsFormsApplication...
Error: Cannot publish because a project failed to build.

The signtool.exe is located at the following location:

%programfiles(x86)%\Microsoft SDKs\Windows\v7.0A\Bin   for x64 system

%programfiles%\Microsoft SDKs\Windows\v7.0A\Bin          for x86 system

Please make sure that the file is available there. Signtool.exe is part of the windows platform SDK. If the file is not present on the system, please install/repair  windows platform SDK OR copy the file from a working system.

Now try to publish the ClickOnce application. If you still find the same issue, please check if you installed the Microsoft .NET Framework 4.5 Developer Preview on the system. The Microsoft .NET Framework 4.5 Developer Preview is a prerelease version of the .NET Framework, and should not be used in production scenarios. It is an in-place update to the .NET Framework 4. You would need to uninstall this prerelease product from ARP. 

4.0beta

After uninstall please reboot the system and install the .Net Framework 4.0 RTM. It can be downloaded from the following link also: http://www.microsoft.com/download/en/details.aspx?id=17718 
Then launch Visual Studio 2010 and publish the ClickOnce Application.