I have a couple of thin clients running, (they are a HP, and an Igel) I replaced the proprietary modules with mSATA SSDs without problems, but as I found myself limited to 2GB RAM, I opted to run lightweight Linux operating systems upon them. The Parky Towers site mentioned by Mike indicated I should be able to run 4GB on the Igel, but I have tried, and not got it to run, so reverted back to 2GB.
I did wonder if it would be possible to harvest the drivers used by the embedded WindowsXP, to re-use with a conventional disk distribution of XP, as I had envisioned that acquiring fresh XP drivers for the thin clients to be unlikely. (Both of them will boot from disk).
I have no idea where to find the relevant drivers, or whether drivers 'harvested' as suggested would work. I would be interested to hear if anyone does know.
(PS the Igel runs Linux Mint fine, as long as I stick to a lightweight desktop: ie not Cinnamon, Mate is iffy, Xfce is fine.)