Check that the installer downloaded correctly by verifying its digital signature (How to check for a corrupted installer or main program).
If the signature is incorrect, please download the installer again and check the new version. (You may need to clear your browser cache, if it keeps downloading an incomplete installer.)
It's also worth checking that your C:\ and temp folders haven't run out of space.
Anti-virus may also interfere with the installer at times, which may be logged by the a/v software if that is the case.