KeyCounter

KeyCounter provides a way of keeping track of when you've been active on your PC during the day. It does pretty much what it says - counts the number of key strokes you make every minute and records that information, along with whether the mouse has moved.
KeyCounter can then be used to browse the files, and display a graph of when you were at your most busy.

It details the time of the first and last keypress in a day, and the graph clearly shows when you weren't at your machine - making the coffee or gone for lunch.
Keep KeyCounter running on your machine all the time, and then call up the history when you want to review your activity - when filling in your timesheet for example.