This problem has been addressed many times in this section and on G1 forums, the majority of the time it is anti-virus related and is also an easy fix. It mainly happens with AVG Avira McAfee but it has happened to me twice this month using Avast, best thing to do is restore the files from your virus vault or re-install the game..
- Do not use Gamersfirst Live for download
- Disable your AV for the download, install and update