Are you logged on as an Administrator?
Run a command prompt with Administrator rights and do:
SFC /scannow
Also, check over this info
HERE and make sure you don't have any of the listed conditions; for one thing I think it says install all the other updates first.
If you still can't install then download SP1 from
HERE - Click more and choose windows6.1-KB976932-X64.exe 903.2MB (since you have the 64bit OS). Then try running that to install SP1.