I looked all over the web for an answer to this and could find little to nothing. I'm using XP Tablet edition and wanted to permanently remove the on screen keyboard as it constantly popped up at inopportune times. There are some reg hacks out there, but mostly they're for Windows Mobile and don't seem to work for XP Tablet.

I take no responsibility if you do this, but what I finally did was to find and rename the KeyboardSurrogate.exe. In my case this was located in the Program Files\Common Files\Microsoft\Ink folder. This is a laptop with a full keyboard and we do not want or use the virtual keyboard functions. This completely removes the on screen keyboard. If you're worried about renaming the exe, you can always copy it off somewhere else or to a USB key so you could restore it if needed.

This eliminated a huge annoyance for me and I hope it helps someone else.