To make the long story short, this doesn't work:
<Icon Id="msiexec.ico" SourceFile="[SystemFolder]msiexec.exe"/>
(Error 4 The system cannot find the file '[SystemFolder]msiexec.exe')
And this doesn't work too:
<Icon Id="msiexec.ico" SourceFile="$(var.SystemFolder)msiexec.exe"/>
Error 3 Undefined preprocessor variable '$(var.SystemFolder)'.