Burning is a big drag on your system & I'm surprised you could watch a movie while burning when you were running WinME. Most people I've talked to can't do much else while burning, other than openning Docs.
Since WinME was an upgrade from 98 it'sless likely to handle the extra load of burning & watching movies at the same time.
As long as you have less than 512megs of memory installed the only thing you can do is shut other running apps off when you burn & want to watch a movie at the same time. But for the time it takes to burn on newer burners I don't really see the need. I can burn a full 700MB CD-R in less than 6 minutes, hardly leaves time to do anything else.