There was an excellent posting on the Tablet PC Buzz forum for doing a clean install of Windows XP Tablet Edition 2005 without requiring the Windows XP Tablet Edition 2005 disk.
Check it out at:
http://www.tabletpcbuzz.com/forum/topic.asp?TOPIC_ID=32497
Disclaimer: I have not tried the outlined reinstall so I can not speak to it's accuracy or any problems it may introduce for the use of your tablet.
Even if you do not do a reinstall, there is lots of useful information contained in the post.