Being blacklisted (list of websites distributing malicious software) has become a common problem among companies worldwide. How is it possible that Google thinks your companies website is trying to infect visitors with viruses when you haven't added or authorized it?
It's important to note, that this article discusses being blacklisted for malicious content that you haven't added yourself. Being blacklisted for using black hat SEO (illegal tactics of search engine optimisation) is out of the scope of this article and quite frankly - your own fault.
Google regularly checks pages it has indexed. If any of them forces visitor to download malicious software, it gets blacklisted. Next time visitors clicks to the link he/she gets a warning informing about possible harm to computer caused by visiting the site. You can imagine most people rather click "back" than "ignore the warning". Damage it can cause to your companies reputation and sales can be huge if the problem is not dealt with quickly.
Initial idea and fear of hacker attack against your website is usually incorrect. In most cases, the attack is not committed by person but worm. Worms spreading by infecting all sites which ftp data it can find from victims computers are common and spread like wildfire.
In sight of previous paragraph, the first thing you should do is scan your computer for viruses. If you don't have antivirus software, but have genuine copy of Windows (worms usually affect only Windows platform), download free Microsoft security essentials by Microsoft. It should be able to remove the worm from your computer.
What to do if you have never logged into your sites FTP server? In that case you should instantly contact your webmaster. It's possible that not only your but all of their clients websites have been effected. They should be notified as soon as possible to avoid major damages. You should notify the company that created the website even if you have logged into the FTP account and found the worm in your computer just to be sure.
Once you have cleaned you computer and website, the last step is letting Google know that the domain is safe once again. For that you must connect your site with Google Webmaster Tools.
If you don't have Google Webmaster Tools account you or your web developer have to create it. It offers number of analytic benefits and the change to fill an application to review your site. You can find guide to setting up Webmaster Tools on the official site, thereby I won't be describing the process in details.
Once you are done, log into Webmaster Tools, select your website profile and click "Diagnostics" in the menu. From there select "Malware" and "Request a review". Google will ask you to give short description of what happened. It's enough to say, that the site was infected by worm and has now been striped of malicious code. After sending the request, Google will remove your site from blacklist within few days. This process can't be accelerated.