A final attribute to consider is Condition. You'll set this to a conditional statement that, should it evaluate to false, the system will display an error dialog and abort the installation. The dialog's text is unchangeable and looks like the following screenshot:
Because the condition is evaluated early on, you can only use built-in variables listed at the following URL:
http://wix.sourceforge.net/manual-wix3/bundle_built_in_variables.htm
However, because you can't change the error message, and the error message always says that the program requires a newer version of Windows, you should really only be checking the operating system version. A variable fit for this purpose is VersionNT. Compare it to an OS version number, formatted as a decimal preceded by v. For example, Windows 7 is Version 6.1. The following example checks that the operating system is Windows 7:
<Bundle Name="Awesome Software"
Condition="VersionNT = v6.1"
...