Our resident expert on Win98 seems to be Foulestroupe. Perhaps he'd replay to a PM(?). He was making noises about switching to something else, but I believe he had Win98 still running recently.

The procedure for getting into BIOS is determined by the computer manufacturer and/or by the chip maker who made the BIOS chip, and there's been a lot of variation in how it's done on different machines.

A likely reason why your DVD doesn't do DVD is that Win98 didn't come with and hence (according to my dim recollection) didn't install the codex files for DVD modes of operation. Checking with the drive manufacturer for updated drivers, drive BIOS, and codex files would be in order, expecially after moving the drive to a new machine. (The drive itself has a BIOS that's separate from the computer BIOS.)

Some optical drive makers also post drive diagnostic programs that may be helpful. Early combo drives had a tendency to fail the DVD part, but still work okay for CDs; so verifying that the DVD functions are working may be needed.

John