Impersonation.cs

public class Impersonation : IDisposable
    {
        public Impersonation(string username, string password, string domain)
        {
            try
            {
                IntPtr tokenHandle; // = IntPtr.Zero;
                Int32 ret;

                if (
                    !LogonUser(username, domain, password, LogonType.Interactive, LogonProvider.Default, out tokenHandle))
                {
                    ret = Marshal.GetLastWin32Error();
                    if (ret != 0)
                        throw new Exception("LogonUser; GetLastError Return Code: " + ret);
                }

                if (!ImpersonateLoggedOnUser(tokenHandle))
                {
                    ret = Marshal.GetLastWin32Error();
                    if (ret != 0)
                        throw new Exception("ImpersonateLoggedOnUser; GetLastError Return Code: " + ret);
                }
            }
            catch (Exception ex)
            {
                throw new Exception("Problem Beginning Impersonation", ex);
            }

        }

        #region LogonUser-Related Enum Values

        /// <summary>Used for the impersonation API calls.</summary>
        private enum LogonType
        {
            /// <summary>Interactive logon type</summary>
            Interactive = 2,

            /// <summary>Network logon type</summary>
            Network,

            /// <summary>Batch logon type</summary>
            Batch,

            /// <summary>Service logon type</summary>
            Service
        }

        /// <summary>Used for the impersonation API calls.</summary>

        private enum LogonProvider
        {
            /// <summary>Default logon provider</summary>
            Default,

            /// <summary>NT 3.5 logon provider</summary>
            WinNT35
        }

        #endregion

        #region Win32 DLL Imports

        /// <summary>Internal windows function used for loging in as a user</summary>
        /// <param name="lpszUsername">The user name of the user to log in as</param>
        /// <param name="lpszDomain">The domain the user is in</param>
        /// <param name="lpszPassword">The users password</param>
        /// <param name="dwLogonType">Method of logging in. <see cref="LogonType"/></param>
        /// <param name="dwLogonProvider">Which provider to log in with.  <see cref="LogonProvider"/></param>
        /// <param name="phToken">Token returned that points to the user.  Needed for impersonation</param>
        /// <returns>False for success True for error</returns>
        [DllImport("advapi32.dll")]
        private static extern bool LogonUser(String lpszUsername, String lpszDomain, String lpszPassword,
            LogonType dwLogonType, LogonProvider dwLogonProvider, out IntPtr phToken);

        /// <summary>Internal windows function used for assuming the identity of a user</summary>
        /// <param name="hToken">Token returned from LogonUser</param>
        /// <returns>False for sucess True for error</returns>
        /// <seealso cref="LogonUser"/>
        [DllImport("advapi32.dll", SetLastError = true)]
        private static extern bool ImpersonateLoggedOnUser(IntPtr hToken);

        /// <summary>Internal windows function used to stop impersonating</summary>
        /// <returns>False for sucess True for error</returns>
        /// <seealso cref="ImpersonateLoggedOnUser"/>
        [DllImport("advapi32.dll", SetLastError = true)]
        private static extern bool RevertToSelf();

        #endregion

        public void Dispose()
        {
            try
            {
                if (!RevertToSelf())
                {
                    Int32 ret = Marshal.GetLastWin32Error();
                    if (ret != 0)
                        throw new Exception("RevertToSelf; GetLastError Return Code: " + ret);
                }
            }
            catch (Exception ex)
            {
                throw new Exception("Problem Ending Impersonation", ex);
            }
        }
    }

Usage.cs

using(new Impersonation("username","domain","password"))
{
  //code to run under impersonated user
}