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Usually a file can be zero if your getting it via torrent as the seeder does not have that part, you can also have files zeroing at random due to a virus or due to the file being infected.
First thing you should do is install avast and do a boot time scan. (uninstall NOD first)
Then after that install AVG 8.5 and leave it running.
There are only 3 possible explanations for your problem.
1) you have a virus.
2) your not downloading the files correctly.
3) your downloading infected files.