Try turning off the Tablet PC feature in Windows. Go to Control Panel > Programs and Features – Turn Windows Features On or Off > uncheck Tablet PC Optional Components. Then reboot.
What about your startup folder?
C:\Users\(User-Name)\AppData\Roaming\Microsoft\Windows\Start Menu\Programs\Startup
Do a complete system search for "osk.exe". It should only find one in the Windows\System32 Folder. If you find more than one, that could be an issue.
Sorry, I meant to type "Windows\ system folders", of which all those are, and not the singular System32 folder I listed. I was looking at something else and typing at the same time. Apologies.
The result I was checking for was it being located in a folder other than a system folder and not actually being the original osk.exe utility, rather an augmented and possibly infected file that also launches the real osk.exe.
At any rate, that doesn't seem to be the case. There has to be something we're missing here...
So, it pops up when Windows loads before you press anything. Then, once you close it, does it ever come back?
If it's any help I believe it started after I told it to launch on startup once, a long time ago. I later turned it off, and it just stayed on. I didn't worry about it too much for a while thinking I would get around to figuring out why, and that's what I'm doing now.