How to Process Start with Impersonated Domain User
public class ImpersonationHelper : IDisposable
{
    IntPtr m_tokenHandle = new IntPtr(0);
    WindowsImpersonationContext m_impersonatedUser;

    #region Win32 API Declarations

    const int LOGON32_PROVIDER_DEFAULT = 0;
    const int LOGON32_LOGON_INTERACTIVE = 2;    //This parameter causes LogonUser to create a primary token.

    [DllImport("advapi32.dll", SetLastError = true)]
    public static extern bool LogonUser(String lpszUsername, String lpszDomain, String lpszPassword,
    int dwLogonType, int dwLogonProvider, ref IntPtr phToken);

    [DllImport("kernel32.dll", CharSet = CharSet.Auto)]
    public extern static bool CloseHandle(IntPtr handle);

    [DllImport("advapi32.dll", CharSet = CharSet.Auto, SetLastError = true)]
    public extern static bool DuplicateToken(IntPtr ExistingTokenHandle,
    int SECURITY_IMPERSONATION_LEVEL, ref IntPtr DuplicateTokenHandle);

    #endregion

    /// <summary>
    /// Constructor. Impersonates the requested user. Impersonation lasts until
    /// the instance is disposed.
    /// </summary>
    public ImpersonationHelper(string domain, string user, string password)
    {
        // Call LogonUser to obtain a handle to an access token.
        bool returnValue = LogonUser(user, domain, password,
            LOGON32_LOGON_INTERACTIVE, LOGON32_PROVIDER_DEFAULT,
            ref m_tokenHandle);
        if (false == returnValue)
        {
            int ret = Marshal.GetLastWin32Error();
            throw new System.ComponentModel.Win32Exception(ret);
        }

        // Impersonate
        m_impersonatedUser = new WindowsIdentity(m_tokenHandle).Impersonate();
    }

    #region IDisposable Pattern

    /// <summary>
    /// Revert to original user and cleanup.
    /// </summary>
    protected virtual void Dispose(bool disposing)
    {
        if (disposing)
        {
            // Revert to original user identity
            if (m_impersonatedUser != null)
                m_impersonatedUser.Undo();
        }

        // Free the tokens.
        if (m_tokenHandle != IntPtr.Zero)
            CloseHandle(m_tokenHandle);
    }

    /// <summary>
    /// Explicit dispose.
    /// </summary>
    public void Dispose()
    {
        Dispose(true);
        GC.SuppressFinalize(this);
    }

    /// <summary>
    /// Destructor
    /// </summary>
    ~ImpersonationHelper()
    {
        Dispose(false);
    }

    #endregion
}

using (new ImpersonationHelper("xxx.blabla.com", "xxxx", "xxxx"))
        {

                if (!string.IsNullOrEmpty(txtFilename.Text))
                    Process.Start(txtFilename.Text);

        }

System.Diagnostics.Process proc = new System.Diagnostics.Process();
        System.Security.SecureString ssPwd = new System.Security.SecureString();

        proc.StartInfo.UseShellExecute = false;
        proc.StartInfo.FileName = "filename";
        proc.StartInfo.Arguments = "args...";
        proc.StartInfo.Domain = "domainname";
        proc.StartInfo.UserName = "username";

        string password = "test";

        for (int x = 0; x < password.Length; x++)
        {
            ssPwd.AppendChar(password[x]);
        }

        proc.StartInfo.Password = ssPwd;
        proc.Start();