click on the link on this page for velofit's Monod spreadsheet (which you can download) - basically slope of work vs time is "long term" CP, and then you can back out CPs for specific time points. You'll need a powermeter or very good estimate of power for any given interval. As much as I like my KK Road Machine trainer, it turns out the published power curve is not that accurate when I benchmark against my PT.

http://velo-fit.com/new/tools.php

I think Golden Cheetah uses the same calculator.