It's possible that the media you used for the previous installs is slightly corrupted - so a fresh download would be a good idea anyhow.
What license type do you have? (Retail Full, OEM, or Upgrade)


Download the SP1Refresh for your language and edition from the links on these pages...

http://www.heidoc.net/joomla/technology-science/microsoft

The links are fordownloads from the Digital River server run for MS.

Create the DVD, and use that for another clean install - this avoids all the problems of updating to SP1 in the first place :)