Hit this like a brick wall, but finally prevailed.
This problem was inherited from a Vista Home Premium x64 upgrade to Windows 7 Home Premium x64, and it sailed right through the upgrade like a hot knife cutting through warm butter.
The on screen keyboard came up prior to any user login, right at boot.
Nothing seemed to make it go away, and I've tried a lot of things that I found on the internet, and none of them ever worked.
Finally decided to chase it down through the registry, using regedit, invoking the find function for "osk.exe"
Found several instances of it, and upon attempting to delete all the OSK keys, I hit the next brick wall, which was a locked registry for this little demon, with or without using admin privileges in regedit.
More gnashing of teeth and a day later, I finally found the method for taking full control of the locked OSK registry keys**HERE** .
Then I went back and searched the registry once again, and every single time I found "osk.exe" I deleted the entire key, and just nuked that thing into oblivion.
Yes, I know I probably also rendered the machine incapable of ever using the on screen keyboard ever again, using such overwhelmingly heavy-handed techniques, but I really do not care, do not expect to ever need the stupid thing anyway, and the satisfaction
of eradicating it from the face of the earth after being annoyed by it for far too long was very rewarding.
It's gone and I'm happy.
If all else fails, maybe try this for yourself and see what happens.