>>921
[DllImport( "shell32.dll", CharSet = CharSet.Unicode )]
static extern int SHGetKnownFolderPath( [In, MarshalAs( UnmanagedType.LPStruct )] Guid rfid, int dwFlags, IntPtr hToken, out string ppszPath );

...
Guid FOLDERID_Downloads = new Guid( "374DE290-123F-4565-9164-39C4925E467B" );

string s;
SHGetKnownFolderPath( FOLDERID_Downloads, 0, IntPtr.Zero, out s );