You can switch DMA on by going to the control panel, and clicking on system properties. On the hardware/hardware devices tab, click on device manager and then open "IDE ATA/ATAPI controllers". Select the channel that your dvd drive is on (primary or secondary) by double clicking on one of them, and then under the advanced settings 'Transfer Mode' make sure you have DMA if available selected, and also auto detection chosen for 'Device Type'.
If you're not sure what channel your drive is on (primary/secondary) or what device (1 or 2), its probably the only one with 'DMA if available' and/or 'auto detection' not selected. I think these are standard settings, so go through both channels and all devices and set the lot to DMA and auto detect :)
You will most likely need to restart your pc for any changes to take effect- good luck!