I have had this exact problem and decided (after doing it easily on a 32 bit machine installed and running perfectly) to slipstream this SP1 with RT 7 (NB slipstreaming only available on 2.6 beta NOT the default, stable version) and spring clean.
It showed "invalid data error" twice so, I am guessing, some versions of this 64bit SP1 may have become corrupted.
I must stress that slipstreaming the 32 bit version was very slow but otherwise painless.
(I used the version from softpedia - but I am going to try another one now)
So far a different (torrent) download is slipstreaming fine...I will try to apply it later.