Step one is to enable DMA support on your ATA66+ hard drives. To get there you'll need to go to the system properties (right click my computer, properties, or go to the control panel and choose system). From there, go to the hardware tab, then the device manager button. Find your IDE ATA/ATAPI controllers. Now, doubleclick on your primary IDE channel, go to the advanced tab, and under transfer mode make sure it reads "DMA if available". Please note that the Current Transfer Mode must read Ultra DMA Mode. If it doesn't...you may as well stop reading, because this tweak will not work for you.
Now, if you only have one harddrive on the primary IDE channel, go ahead and select none for the second channel shown here. It's not used, so it just wastes time during the O/S boot. With that done, ok out of the properties and do the same thing for the secondary IDE controller. Once more, if you have no devices, or only one device, connected to this controller, go ahead and turn the unused channels to none.
Now for the tweak portion. As you may have surmised, we will need to run regedit by going to start | run and typing regedit, then hitting enter or ok. Navigate to the following key:
The key may actually vary slightly. Rather than 0000, it may display as 0001, 0002, etc. Whatever you have, select your key(s). Now, create a new DWORD value and name it "EnableUDMA66", and give it a value of 1. If, for whatever reason, you'd like to disable this setting later, you can change the value to 0.
And that is it. Reboot the computer for your changes to take effect.