Thanks Neil. SiS has released some new drivers for the 648 Chipset (648 + 963) - both certified under Windows XP

IDE Driver r1.12
AGP Driver r1.01

The USB 2.0 driver is the same as the one supplied in Windows XP SP1.

Since installing the new IDE driver Windows XP correctly shows my ATA133 drive as DMA Mode 6.

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Individual download links and mirrors on the above page.