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.