The download is probably corrupt and it's probably easiest to download again. If you are data conscious (due to ISP, time, or slow download) there's a few checks you can do before downloading again:
- See if the digital signature is intact (file properties > Digital Signatures > click details)
- Compare checksums with someone else:
I just downloaded and here's my MD5/SHA1 sums for comparison:
(if you are unsure what this is, google for utility that will generate the sums for you. I use a shell integrated one, 'HashCheck')
Thank you Ozz; that worked.