Hi greybyrd, welcome to CDFreaks!
I would have chosen the General Software forum instead, but please don't make a new thread.
Nero shouldn't use 100% CPU for burning, but if you use Nero Recode, NeroVision Express or some similar application that encodes, recodes or transcodes video into DVD Video format (before burning) then those applications will probably use all the CPU they can get, and there's no way to change that.
There is a way to deal with the consequences, however. If you open up Task Manager (Ctrl-Alt-Delete) in Windows 2000/XP, go to the Processes tab, click twice on the CPU column to sort by CPU usage in descending order, then your CPU hungry application should be at the top. Then you can right-click on that process and Set Priority to BelowNormal or Low. That will give priority to your other tasks so that your pc will feel faster even though it's using all available CPU power.
It's not a good idea to have the Priority at BelowNormal or Low when performing an actual burn however, so you might want to do conversion first and then burn later.