Well, hindsight is 20/20
The Compaq and HP FAQ on this site indicates that the ze5000 series (all models) have very problematic bios updates, and of course, I didn't find that out until I had a problem...
Here is my logfile from the Phlash utility
edit: it looks as if it completed writing to 7 out of 8 blocks
Did my bios update actually fail? I ran the Windows flash utility (never again) and although the progress bar showed successful completion of all steps, the system shut down abruptly without a confirmation dialogue.
Now I can still power on the lappy, boot into the bios utility, and RUN UBUNTU LIVE CD without any obvious problems. However, when starting Windows XP Home, it freezes on the screen with the Windows logo and progress bar. Restarts don't help, it won't let me run in safe mode, last good configuration freezes too, and it also freezes when booting from a Win XP Pro retail disc (downloaded from torrent site because my oem disc is unavailable) so I can't run "repair xp installation" utility.
Fortunately, from Ubuntu I was able to mount the ntfs drive and retrieve important files, and it looks like no important files are missing. I also copied the whole bios folder (bios.bak, phlash.exe, etc). So perhaps my boot sector is corrupt?
I have no formal computer science education, just a student with some experience. Hopefully you wise fellows can point me in the right direction. I've read many helpful posts at this forum, including the one about the crisisdisk on the hp support forums, and I'd really appreciate any ideas before I get myself any deeper.