To enable DMA for my DVD and CD-RW I had to "disable" these two drives in BIOS (I simply selected "None" instead of "Auto"). After that I was able to use DMA in XP, without installing VIA drivers that is! Don't know if it will help you, but it's worth trying I guess.
this fix is for everyone using Windows XP and can't enable DMA for CDROM or hard disk. If you are sure that your cdrom supports DMA mode then run regedit, press "Edit--Find" and there put the word "timingmode". Regedit will search the registry and find the two ide buses. You will recognize these easily. It is written primary and secondary, master and slave. In my case my Toshiba DVD was running in PIO mode instead of DMA and was located in primary slave. I changed the value of two strings including the TimingMode for primary slave. The value for DMA33 is 8208 (decimal). Furthermore the second string must be ffffffff. I found every string with TimingMode that refered to primary slave and made these changes. After restarting Windows DMA33 was enabled!!!