This morning, after Windows Update, our builds started failing with: Task could not find "AL.exe" using the SdkToolsPath "" or the registry key "HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Microsoft SDKs\Windows\v7.0A".
It turns out that Microsoft, in their wisdom, decided to change the default version of .NET to be used by msbuild from the Windows SDK to 3.5 SP1.
The fix was easy when I realised what was up. Start -> Windows SDK Configuration Tool -> select v7.1 (as all our apps are .NET 4.0) -> Make Current.
Just thought this might save you an hour of hassle.