Thanks for all the replies.
After further testing, I think my DVD writer may be broken. When I try to explore the Roxio DVDs (to check file names, etc.), Windows XP returns a message saying "disk is not Formatted, cannot read from this disk."
And now when I copy regular files to a DVD using Nero, I get the same issue, even though Nero verifies the copy job. Strange.
I'm going to try a new DVD writer.