The iso file is intact. (And I downloaded it on Windows and checked the checksum on Linux, so it doesn't look like a character mapping problem.) But someone said something about checking whether the file had burned to the DVD correctly. This was the bit I couldn't work out how to do.
Someone said I could run a check from the CD's boot menu. But I can't find the boot menu. I don't even know what the filename extension for batch files in Linux is. It said that running from boot doesn't work on my computer, and offered the option (which I accepted) of modifying the hard disk's boot sector so that there's a menu at startup where I choose between Windows and Ubuntu-from-CD. Anyway, the upshot is if someone can help me find the boot file then maybe I can run it (or part of it) to check the integrity of the CD.
The file was ubuntu-10.04-desktop-i386.iso . How do I know if this is the right file? How badly would I expect it to go wrong if I got the wrong iso file? At the moment, Linux sort of works, it's lost most of the previous bugs but picked up some new ones, and it crashes quite a lot.