CreateProcessAsUser

[StructLayout(LayoutKind.Sequential)]
        public struct SECURITY_ATTRIBUTES
        {
            public int nLength;
            public IntPtr lpSecurityDescriptor;
            public int bInheritHandle;
        }

        [StructLayout(LayoutKind.Sequential)]
        internal struct PROCESS_INFORMATION
        {
            public IntPtr hProcess;
            public IntPtr hThread;
            public int dwProcessId;
            public int dwThreadId;
        }

        [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
        struct STARTUPINFO
        {
            public Int32 cb;
            public string lpReserved;
            public string lpDesktop;
            public string lpTitle;
            public Int32 dwX;
            public Int32 dwY;
            public Int32 dwXSize;
            public Int32 dwYSize;
            public Int32 dwXCountChars;
            public Int32 dwYCountChars;
            public Int32 dwFillAttribute;
            public Int32 dwFlags;
            public Int16 wShowWindow;
            public Int16 cbReserved2;
            public IntPtr lpReserved2;
            public IntPtr hStdInput;
            public IntPtr hStdOutput;
            public IntPtr hStdError;
        }

        public enum SECURITY_IMPERSONATION_LEVEL
        {
            SecurityAnonymous,
            SecurityIdentification,
            SecurityImpersonation,
            SecurityDelegation
        }

        public enum TOKEN_TYPE
        {
            TokenPrimary = 1,
            TokenImpersonation
        }

        [DllImport("advapi32.dll", CharSet = CharSet.Auto, SetLastError = true)]
        public extern static bool DuplicateTokenEx(
            IntPtr hExistingToken,
            uint dwDesiredAccess,
            ref SECURITY_ATTRIBUTES lpTokenAttributes,
            SECURITY_IMPERSONATION_LEVEL ImpersonationLevel,
            TOKEN_TYPE TokenType,
            ref IntPtr phNewToken);

        [DllImport("advapi32.dll", SetLastError = true, CharSet = CharSet.Auto)]
        static extern bool CreateProcessAsUser(
            IntPtr hToken,
            string lpApplicationName,
            string lpCommandLine,
            ref SECURITY_ATTRIBUTES lpProcessAttributes,
            ref SECURITY_ATTRIBUTES lpThreadAttributes,
            bool bInheritHandles,
            uint dwCreationFlags,
            IntPtr lpEnvironment,
            string lpCurrentDirectory,
            ref STARTUPINFO lpStartupInfo,
            ref PROCESS_INFORMATION lpProcessInformation);

        public static Process CreateProcessAsUser(string filename, string args)
        {
            IntPtr hToken = WindowsIdentity.GetCurrent().Token;
            IntPtr hDupedToken = IntPtr.Zero;

            PROCESS_INFORMATION pi = new PROCESS_INFORMATION();
            SECURITY_ATTRIBUTES sa = new SECURITY_ATTRIBUTES();
            sa.nLength = Marshal.SizeOf(sa);

            DuplicateTokenEx(hToken,
                (uint)0,
                ref sa,
                SECURITY_IMPERSONATION_LEVEL.SecurityIdentification,
                TOKEN_TYPE.TokenPrimary,
                ref hDupedToken);

            STARTUPINFO si = new STARTUPINFO();
            si.cb = Marshal.SizeOf(si);
            si.lpDesktop = string.Empty;

            string path = Path.GetFullPath(filename);
            using (WindowsIdentity.Impersonate(IntPtr.Zero))
            {
                CreateProcessAsUser(hDupedToken,
                    path,
                    string.Format("\"{0}\" {1}", filename.Replace("\"", "\"\""), args),
                    ref sa,
                    ref sa,
                    false,
                    0,
                    IntPtr.Zero,
                    @".\",
                    ref si,
                    ref pi);
            }

            return Process.GetProcessById(pi.dwProcessId);
        }