Press Enter while at same time starting the stop watch. Record the time on the stop watch. This is the system's Post-Logon time. Monitors have been observed to effect BIOS POST and Pre-Logon time. Testing on systems that include a network adapter should be done on a small private network.
Network traffic can cause large deviations in measuring boot and resume from hibernate times.
Many BIOSes beep before handing off to the operating system. Using the Windows Me F8 Boot menu as the timing point ensures that the full BIOS POST time is being measured. Windows Me changes the way the shell loads so that the user can start using the PC before all items have loaded during boot. In Windows 98, you needed to wait for every item in the Run key and the Startup group to fully execute before the PC was usable. In Windows Me, you can start using the PC before all these programs have loaded and the hard disk has settled.