A similar question titled How do I log startup and shut-down times in Windows 7? mentioned that PC startup and shutdown times can be recorded in the event viewer.
Events logged in the viewer are only recorded from August 2011 (despite the fact the computer has been used since 2008). The raw data is also difficult to visualise without exporting it and viewing the data in another program such as Excel.
I've searched Google for software but I can only find search items related to desktop assistance and parental security.