Mikev,
Saw your post, not sure if you have figured it out yet or not but i had the same issue on an S90. Wyse tech support gave me the following instructions. Hopefully they will work for you. I never got to try it out, but it sounds like it should work.
XPe – setting the TEMP and TMP to a mapped drive
Scenario – when installing S/W onto an XPe thin client and it runs out of space on either the TMP or TEMP folders
1. On a server, share a folder.
a. Under this shared folder, create a TEMP folder and TMP folder
2. On the XPe client, log in as default Administrator
3. Disable Write Filter. The XPe thin client will reboot
4. After reboot, log into the XPe thin client as default Administrator
5. Map to the shared drive on the server (from step 1)
a. Right click on My Computer -> Map Network Drive
6. Change the XPe Environment Variable for TMP and TEMP on the XPe thin client to the mapped drive
a. Right click on My Computer -> Properties -> Advanced -> Environmental Variables
b. Highlight TEMP -> Edit. Change from z: to mapped drive letter in Step 5
c. Highlight TMP -> Edit. Change from z: to mapped drive letter in Step 5
7. Install the S/W
8. Test the S/W
9. Change TEMP and TMP back to the z: drive
10. Enable Write Filter