Journal du Net article reports that about 8% of Windows sessions require a machine reboot. The relevant quote (translated from french) is: "The average rate of failures requiring a system reboot has been measured at around 8% per session. This number varies widely depending on the version of Windows. Windows 2000 has a failure rate of 4%, and NT4 is at 3%, whereas Windows XP is close to 12%." The study was originally made by Acadys and Microcost and gathered data from 1.2M machines belonging to about one thousand companies over a period of one month in seven different countries."