This function will use the test-key function posted earlier. It will check which .NET frameworks are installed (currently only checking for .NET 4.0) but can be easily adapted and/or expanded.
function Get-Framework-Versions()
{
$installedFrameworks = @()
if(Test-Key "HKLM:\Software\Microsoft\NET Framework Setup\NDP\v4\Client" "Install") { $installedFrameworks += "4.0c" }
if(Test-Key "HKLM:\Software\Microsoft\NET Framework Setup\NDP\v4\Full" "Install") { $installedFrameworks += "4.0" }
return $installedFrameworks
}