You can try to use Environment.TickCount to determine how long ago a machine was started. But Environment.TickCount returns a 32-bit signed integer which can overflow for systems that stay up for days at a time.

To avoid this overflow problem, you can query the "System Up Time" performance counter:

public TimeSpan SystemUpTime()
{
  PerformanceCounter upTime = new PerformanceCounter("System", "System Up Time");

  // You've got to call this twice. First time it returns 0 and the second time it returns the real info.
  upTime.NextValue();
   
  return TimeSpan.FromSeconds(upTime.NextValue());
}

Caveat: if your software runs with limited privileges you may not be able to query the performance counter. For example, I run my website on a shared server, and it's not allowed to query this performance counter :-(.

This tip is based on a newsgroup posting by Stoitcho Goutsev.