On my home laptop (Win 8.1) there's something in the Task Manager that shows in HH:MM:SS how much "Idle" time so from that I usually get an idea of how long since I last booted (based on CPU quad core, or whatever).
But at work seems like Task Manager in Windows 7 does not show that task for some reason?
So I did a quick Google and found out you can just go to CMD and type SYSTEMINFO and after a few moments of info-gathering you see a line "System Boot Time:" (as a date/time value) ... in Windows 7 you apparently see a line "System Up Time:" (in Days, Hours, Minutes, Seconds -- instead of a date/time value).
Found via http://www.howtogeek.com/80068/easily-determine-windows-uptime-in-windows-7-vista-or-xp/ (I feel a bit "behind the times" though, been around a long time based on https://stackoverflow.com/search?q=%5Bwindows%5Dsysteminfo )
(PS: another way is to simply go to the "Performance" tab in Task Manager -- below the # of threads/processes, and above the size of Page File, you can see the "Up Time" value in DD:HH:MM:SS)
Most workstations will have a max up time of 30days due to Patch Tuesdays...
Patching takes all the fun out of up-time at the OS level.