UpTime.Net

 1 PerformanceCounter pc = new PerformanceCounter("System", "System Up Time");
 2 
 3 //Normally starts with zero. do Next Value always.
 4 pc.NextValue(); 
 5
 6 TimeSpan ts = TimeSpan.FromSeconds(pc.NextValue());
 7
 8 MessageBox.Show(Environment.MachineName +
 9    " has been up for \n\n" +
10     (ts.Days < 1 ? "" : ts.Days + " days, ") +
11     (ts.Hours < 1 ? "" : ts.Hours + " hours, ") +
12     (ts.Minutes < 1 ? "" : ts.Minutes + " minutes ") +
13     " and " + ts.Seconds + " seconds.");