Just keep going with it. Even a watch with a second hand you have to interrupt to have a look at it, you so the computers timer is enough.
An alternative method - if you have a tape recorder get a blank tape using your computer's second hand record the word 'start'. Watch the time on the computer while recording and say 'stop' when 45 seconds are up.
In all super accuracy isn't the point - its the general experience. So just keep going.
Alex
[This message has been edited by AlexK (edited January 25, 2003).]