Carefulness is the best anti-virus software

The safe Internet environment won’t be back. The Internet has become a successful tool for hackers to spread their “new works”.The world seriously realized that there was a type of virous virus which culd could spread very quickly throughout an organization. It wasn’t unusual to find hundreds of computers within a personal company infected by viruses transmitted via e-mail attachments.
In this case [OR: As a result] , anti-virus software was created to fight with the viruses. Companies invest in anti-virus software equipped to with their computers. They couldn’t have more confidence in protecting against viruses ["protection against viruses" is also acceptable]. **Their complacency will soon turn out to be a big mistake.** Powerful viruses have an incredibley incredible update speed which anti-virus software can’t follow.
We can easily be attacked by viruses even when we are equiped equipped with the best software in the world.The security of our computers is only as strong as the weakest link. The real reason is the human factor. Downloading unauthorized software or open some strange webside website is where the viruses come from.
In a short word [OR: In conclusion], *carelessness* [*OR: being careless]  while using the computer is the reason why our computer system is like the weakest link [I'm a little confused here. . In another word other words, if you are worried about lack of money to buy the best anti-virus software,do remember that carefulness is the best anti-virus software.

**You can rewrite this sentence as this: "Their complacency, however, will soon turn out to be a big mistake.