What is this DMA thing and how do I enable it?
DMA stands for Direct Memory Access.
In reference to DVD, DMA is used to transfer the DVD audio/video without using the CPU. This gives the CPU more time to decode the data. Without enabling DMA, your computer will suffer a 50% CPU hit.
To enable DMA do the following:
Right clicking "My Computer", select "Properties" > "Device Manager Tab". Get to the "DVD-Rom drive device" and click "Properties".
You should have a DMA checkbox option. Enable it.
You may get a warning telling you that this may cause problems with your hardware. If it does cause a problem (I doubt it will), turn it back off.
If you don't see a DMA checkbox, it may mean you are running an old Windows 95. In that case I recommend upgrading. It may also mean that you are running a Super-7 motherboard. In that case, I recommend getting the latest IRQ routing drivers for your motherboard (from the motherboard manufacturer).
Under Windows 2000/XP, the DMA checkbox may appear under the Hard Disk Controllers section of the Device Manager.
This entire question doesn't apply to any SCSI drives, as they don't use the DMA interface.