For Windows Vista and newer there are accessibility tools on the lower left corner of the screen.
Windows XP however needs a registry edit to show the on-screen keyboard on the login screen.
Run regedit and navigate to:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\WindowsNT\CurrentVersion\Winlogon
Edit the System key and enter osk.exe and click OK.
The on-screen keyboard should appear at the next login.