FCP 4.x and FCE 2.x have a conflict with Norton anti-virus and virex which can lead to freezes and other problems with the "Capture Now" command. The typical scenario is that FCP will freeze while capturing and you must force quit the application. After this crash, a new file with the .av extension will appear on your drive, probably in the current capture scratch folder.

This file will appear to use all but 2GB of the free space on your hard drive, so the next time you try to capture in FCP you'll get an error message saying that you don't have enough space on your hard drive. The solution is two fold: (1) throw away the offending .av file, and (2) be sure that Norton and Virex are turned off before capturing in FCP. See the following Apple tech note: http://discussions.info.apple.com/webx?13@686.hcwdaRV1SKF.0@.68a84950/0

It appears that other sorts of crashes can also create files which appear to eat up all your hard drive space. See this post from Cleveland Brown where he forwards some instructions from Apple Tech Support on correcting problems with a corrupted preference file:

http://www.dvinfo.net/conf/showpost.php?p=310257&postcount=6