The t5325 has no BIOS. Instead it uses the bootloader uBoot.
The website for uBoot is here. On the face of it these pages are written for people who know all about uBoot and just want to use it. It takes a little bit of finding but there is a presentation that almost tells you something about uBoot.
A brief explanation I found elsewhere is:
U-Boot is a boot loader program that is usually stored in flash memory on the embedded system. U-Boot can load files from a variety of peripherals such as a serial connection, an ethernet network connection, or flash memories. U-Boot is the program that gets executed at system reset which will automatically load up another application (such as the Linux kernel). U-Boot can even parse many types of filesystems on many types of storage devices.
To do anything with uBoot you'll probably have to use the serial interface on the board. At the time of writing this is a future project for me. I've ordered some Micro JST connectors and a serial-to-USB interface for not-a-lot from Hong Kong. They'll get here eventually!
The t5325 runs HP's ThinPro operating system which is essentially Debian.
If it hasn't been changed the default administrator password is root.
There is a lot of information available for download from the HP website. One interesting document is the "Troubleshooting Guide: HP t5325 Thin Clients" which covers such topics as backup/restore and USB booting.
Any comments? email me. Last update July 2011