I don't get it vista won't install

I get all the way done with the installation 3 out of 3 steps 100% done. After a while of finalizing everything I guess it has some kind of error and completely reverts. If someone else already got SP1 with the same cd is that possibly why? And if so, why won't it just tell me that?

Is it a clean install? Or service pack install? Are you doing it from Windows updates?