No windows 7 e 8:
System.OperatingSystem osInfo = System.Environment.OSVersion;
string keyName = "HKEY_CURRENT_USER\\Software\\Microsoft\\Windows\\Shell\\Associations\\UrlAssociations\\http\\UserChoice";
tExpand = (string)Registry.GetValue(keyName, "Progid", "Default if TestExpand does not exist.");
No XP o registo é em: HKEY_CLASSES_ROOT\\http\\shell\\open\\command
Onde está "Progid" não colocar nada.