Administrative Name And Password It Tells Me Cannot Log Me In

had to install xp home over xp pro. all the files and user folders that were there before i did this are still intact,but.............. when i push control alt delete to bring up the administrator that works but when i go to type the administrative name and password it tells me cannot log me in. is there like a default password(did not have to type one in when i installed xp home,just a name and company)or some way to find out what it is