How can I find all programs with a GUI (not just arbitrary windows) that are running on my local machine?

You could use EnumWindows with p/Invoke, but using the static Process.GetProcesses() found in the System.Diagnostics namespace will avoid the interop overhead.
[C#]
Using System.Diagnostics;
...
foreach ( Process p in Process.GetProcesses(System.Environment.MachineName) )
{
if( p.MainWindowHandle != IntPtr.Zero)
{
//this is a GUI app
Console.WriteLine( p ); // string s = p.ToString();
}
}

[VB.NET]
Imports System.Diagnostics
...
Dim p As Process
For Each p In Process.GetProcesses(System.Environment.MachineName)
If p.MainWindowHandle <> IntPtr.Zero Then
'this is a GUI app
Console.WriteLine(p) ' string s = p.ToString();
End If
Next p


There is one potential problem on Windows 98. If a process was started with ProcessStartInfo.UseShellExecute set to true, this MainWindowHandle is not available.