XP can`t find license at start up

When XP Home boots up it gets as far as the welcome screen and then displays a message saying “a problem is preventing Windows from accurately checking the license for this computer error code 0x80040017”.
This computer was working fine and as far as I`m aware (it`s my MD`s machine not mine) nothing has been installed or fiddled with.
On checking the MSKB site the only problem like it (310794)has a different error code and refers to a change of security providers. On booting into safe mode the registry has no entries for cryptography in HKEY_USERS and the drive letter hasn`t been changed.
Using the roll back feature also doesn`t work.
ANY ideas??