FRAPS is a good tool to do that. It will record in uncompressed AVI so the files are very large. You may want to record to a secondary HDD because FRAPS will be writing a lot of data, slowing the readout significantly. Other than that i found it to be the easiest to use and very cheap. Though it is capture only. So you will need another program for editing.
The free version allows you to only record 30 second clips with a watermark added. That limitation isn't present in the full version.