The latest updates for Spybot (most dated 3 November) seem to recognize some key Tablet PC functionality as a threat and delete it. The damage can be undone with Windows XP System Restore.
Spybot detects what it refers to as "Smitfraud-C.Toolbar888", and flags the following registry entries as problems:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\WinLogon\Notify\TabBtnWL
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\WinLogon\Notify\Sebring
It offers to fix the "problem", and if you then re-boot you find that the "Change tablet and pen settings" icon is missing from the tray and many Tablet buttons are disabled (on Motion Computing LS800 the Escape, Function, 5-way directional control button, Motion Dashboard button and Rotate Display button, yet the programs seem to launch properly if invoked by clicking on shortcuts).
System Restore to a time immediately before running Spybot fixes the problem.
I've reproduced this problem in a case in which the only item I allowed Spybot to fix was "Smitfraud-C.Toolbar888".