In Event Viewer look inside the System events.
Look at the time column, (2nd column from the left) and the Source column, (4th col from the left). Scroll down to look for gaps between two "eventlog" entries.
The last entry of the previous shutdown will be eventlog. Double click it to open up its Properties window and it will say "The Event service was stopped". This is when the computer was shut down.
Look above that for two more eventlog entries. The top one will say "The Event log was started". That is when the computer was started up.
Using the combination of date, time and source column entries you can work out when the computer was booted up and shut down.
Mark