I put together a simple Excel timesheet that does a pretty good job of keeping track of tasks while working on the computer. It acts as a timer by allowing you to enter start and stop times.
For date, keyboard shortcut is Ctl + ; (semicolon key)
For time, keyboard shortcut is Ctl + Shift + : (colon key)
When I start a task, I enter date, client, and then go over to the start cell and enter the time shortcut above. Then I tab over to the stop cell. When I stop, I enter the time shortcut again. If more than a minute has passed, the elapsed time will read in tenths of hours.
The time is automatically totalled. If you want to add rows, remember to copy the entire row above into the new rows to preserve the formulas.