Process start error when using credentials ("The handle is invalid")

I have an Windows Form application that supplies the User Name, Domain,
and Password to the StartInfo, and it throws this:

System.ComponentModel.Win32Exception: The handle is invalid at
System.Diagnostics.Process.StartWithCreateProcess(ProcessStartInfo
startInfo) at System.Diagnostics.Process.Start()

When I allow the credentials to default to current user I get no such
error, and the process I start works to the extent that it doesn't need
to use credentials (the creds are necessary for mapping a drive in an
MSBuild script). Here's the code that fills the start info:

Process p = new Process();
ProcessStartInfo si = new ProcessStartInfo("MSBuild.exe",
"TheMSBProj.proj");
si.WorkingDirectory = msBuildWorkingDir;
si.UserName = txtUserName.Text;
char[] psw = txtPassword.Text.ToCharArray();
SecureString ss = new SecureString();for (int x = 0; x < psw.Length;
x++)
{
    ss.AppendChar(psw[x]);
}
si.Password = ss;
si.Domain = "ABC";
si.RedirectStandardOutput = true;
si.UseShellExecute = false;
si.WorkingDirectory = txtWorkingDir.Text;
p.StartInfo = si;
p.Start();

It isn't that the user/psw isn't matching, because when I provide a bad
psw, for example, it catches it. So, this "invalid handle" thing is
happening after the cred is passed. Any ideas on what I might be
omitting or screwing up?

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