Thanks to both you guys I've ended up with this:
string servicename = Microsoft.Win32.Registry.LocalMachine.OpenSubKey(@"SYSTEM\\CurrentControlSet\\Services\\EventLog\\Application\\").GetSubKeyNames().Where(s => s.StartsWith("CardSpace")).FirstOrDefault();
string dotNetFourPath = "SYSTEM\\CurrentControlSet\\Services\\EventLog\\Application\\" + servicename;
using (RegistryKey registryKey = Registry.LocalMachine.OpenSubKey(dotNetFourPath))
{
Console.WriteLine(registryKey.GetValue("EventMessageFile"));
string sDt = registryKey.GetValue("EventMessageFile").ToString();
string[] sVar = sDt.Split('\\');
string sYourValue = sVar[4];
Console.WriteLine(sVar[0]);
Console.WriteLine(sVar[1]);
Console.WriteLine(sVar[2]);
Console.WriteLine(sVar[3]);
Console.WriteLine(sVar[4]);
Console.WriteLine(sVar[5]);
Console.WriteLine(sYourValue);
}