At work, we've been working with the HP t55xx series thin-clients, mainly for one of our customers.
This article contains some things we've found to be very useful. It is not a real howto, but rather a collection of smaller items, telling you how to do certain things, and those things are not easy to find in the manuals.
There are 4 ways to reset a t5530 to factory defaults:
If method 2 (pressing F9) does not work, and you are sysadmin, you can re-enable it by going into Control Panel, Security, then verify the Enable Reset Hotkey is selected on the System tab. If it's checked, the reset will work by holding F9 at boot.