The OS drivers simply allow a DVD drive to work as a CD-ROM drive in the system.
You need the DVD decoder software, not the drivers. If your system shipped without a PCI card decoder, it shipped with a software decoder - probably Ravisent's Cinemaster, but also possibly WinDVD. The installs for these should be on the driver CD you received with your system.
If they weren't, you will probably have to buy a copy - they're not freeware, so they're generally not available for download. WinDVD is relatively inexpensive ($40 or so) and a decent performer. As far as I know, Ravisent doesn't sell to end users - just to OEMs.
When I had to redo my Quantex a few months ago the DVD software was on the driver CD. It was listed by itself under Cinnemaster. I hope that this will help you some.
I need to find the DVD drivers for the Quantex DVD. The one that they included is a Toshiba sc-D2402. All I can find says it should be recognized by win98 and it is but cannot play DVD movies. Any help is appreciated.