Sounds like it might be a DMA (Direct Memory Access) problem. It could be that your DVD drive and/or hard drive are not using the fastest access mode (UDMA-2 is usually the fastest mode for cd/dvd drives).
Look at this page for help. http://winhlp.com/WxDMA.htm
Specifically, look at the section titled "Check Your IDE Port Mode". If your drives are in PIO mode, you need to follow the remedy on that page to fix it.
BTW, it's usual to see a dual core, or dual processor, show 50% usage in the Task Manager. Windows reports the usage as the average of all the processors, so in typical programs only one core is being used at a time (100+0)/2 = 50. Basically, one core is pegged at 100% usage and the other is waiting for another program to utilize it. You would probably see the processor usage go up if you open another program while burning.