میتونی با عبارات منظم (Regular Expression) بدست بیاری:
string str = "Username: 1 Password: 1 Expiration: 14/08/13 Username: 2 Password: 2 Expiration: 14/08/13 Username: 3 Password: 3 Expiration: 14/08/13";
Regex r = new Regex(@"Username: ([a-zA-Z0-9!@#$%^&*<>?,\./\{\}\[\]:"";']+) Password: ([a-zA-Z0-9!@#$%^&*<>?,\./\{\}\[\]:"";\']+) Expiration: [0-9]{2}/[0-9]{2}/[0-9]{2}", RegexOptions.IgnoreCase);
foreach (Match m in r.Matches(str))
{
listBox1.Items.Add(m.Groups[1].Value + "\t:\t" + m.Groups[2].Value);
}