You might already be in DMA mode.
To check, or to turn on DMA,
Click: Start, Settings, Control Panel, System, Hardware, Device Manager, IDE ATI/ATAPI Controllers, Primary IDE Channel; Right-click for properties, Advanced Settings, Device Type "auto Detection", Transfer Mode, Device 0,(Select DMA If Available), do same for Device 1. Do same for the secondary channel device 0 and device 1. If the "Device Type" is greyed out, there is no drive on that channel. Click Ok.
If DMA is available, selecting or leaving the drive set to "PIO" will make the device perform much slower.
Your BIOS should also be set to "Auto Detect" or the DMA mode specified on the drive sticker if the BIOS can't auto detect. The DMA mode on the sticker may be anything from DMA Mode 1 to DMA Mode 6.