How do I retrieve the username that a Windows service is running under?

WMI is your friend. Look at Win32_Service, specifically the StartName property. You can access WMI from C# via the System.Management.ManagementClass.

If you've not used WMI before, this article seems to be quite a good tutorial.

You can find this using the Windows Registry, reading the following string value, replacing [SERVICE_NAME] with the name of the Windows Service:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\[SERVICE_NAME]\ObjectName

Using WMI, with the System.Management you can try the following code:

using System;
namespace WindowsServiceTest
{
    class Program
    {
        static void Main(string[] args)
        {
            System.Management.SelectQuery sQuery = new System.Management.SelectQuery(string.Format("select name, startname from Win32_Service")); // where name = '{0}'", "MCShield.exe"));
            using (System.Management.ManagementObjectSearcher mgmtSearcher  = new System.Management.ManagementObjectSearcher(sQuery))
            {
                foreach (System.Management.ManagementObject service in mgmtSearcher.Get())
                {
                    string servicelogondetails =
                        string.Format("Name: {0} ,  Logon : {1} ", service["Name"].ToString(), service["startname"]).ToString();
                    Console.WriteLine(servicelogondetails);
                }
            }
            Console.ReadLine();
        }
    }
}

You can then later substitute the commented code with your service name, and it should only return the instances of your service process that is running.