On-Screen Keyboard in Windows 7 can be replaced with other on screen keyboard by replacing the file C:\Windows\System32\osk.exe
(First, need to take ownership
Take Ownership Shortcut)
For test purposes I have tried that with freevirtualkeyboard.com and it worked in windows and in Log in screen.
(I would NOT recommend freevirtualkeyboard.com - it goes on it's website each time you start it)