SNMP OID: .1.3.6.1.2.1.1.3 [for Unix, Linux, Windows as well as many other operating systems]
.iso.org.dod.internet.mgmt.mib-2.system.sysUpTime
Description The time (in hundredths of a second) since the network management portion of the system was last re-initialized.
 

WMI Class: Win32_PerfFormattedData_PerfOS_System [Win2003 only]
Property: SystemUpTime
Description: System Up Time is the elapsed time (in seconds) that the computer has been running since it was last started. This counter displays the difference between the start time and the current time.