I met a woman at a talk I gave who kept a little pad on her desk and made a note every 15 minutes about what she was doing so she could bill her clients accurately. Most of the other consultants in the room (who generally work at home) looked at her in awe and then at each other guiltily and admitted that they guesstimate their billable hours.
There is a way to review in excruciating detail what you've done on the computer all day. It's a small, free program called Time Snapper. Its main purpose is to help people fill out their time sheets.
What it does is take pictures of your screen every few seconds (you can vary the time, as well as the playback speed). It runs in the background unobtrusively and you can browse your recent activity any time. I found it rather eerie to watch a fast motion movie of my every screen movement, including words I wrote popping across the page.
In addition to being a fairly effortless way of keeping track of computer time, it was interesting to me to watch my thought processes. During one recorded sequence, I wrote a few emails, but I kept switching away from the composition screen to check information on the Internet as I wrote.
I know that I do this, but seeing it happen really made me pay attention to how long it can take me to dash off a quick email. Time Snapper can be a useful tool for discovering why it takes you more time than you predicted to do a given computer task, and therefore be more realistic when you're budgeting time.
Comment Preview