I narrowed down the problem. It will boot from an XP install CD, actually a DVD. This boots without emulation. The discs that fail to boot, these are DVDs also, use floppy emulation. These discs boot okay in my other computers, including my Intel DP35DP.
The BIOS has 4 options for USB mass storage emulation. None work, including the default AUTO. My DP35DP mobo uses AUTO and it will boot these discs without a problem.
P.S. It would be nice if I could copy and paste this text. Where did they get this forum software, on eBay? A preview button wouldn't hurt either.
My previous post needs some clarification. My DP35DP that boots these DVDs has the optical drive connected to the IDE port. Maybe my DG43NM would boot them with such a connection, instead of the USB connection used currently.