Well, looking at the fact that your are wanting to install SP2 which is in beta I'd say you probably have more Beta software installed. Henceforth Vista is having trouble as it is all untested beta rubbish. No use installing it unless you are a developer or just have too much time at hand to resolve and report bugs.
Sounds though like the download corrupted. Try downloading on another PC, chuck it on a stick or CD, and install from there.
Other possible reasons could be corrupted windows OS files (Not unusual with beta's) bad HDD (Run a check disk, and please do not use reg fixes) or bad lan connection if installing from network share.