What are viruses and worms?
Viruses and worms are self-replicating, malicious codes that attach to an application program or other executable system components and leave no obvious signs of their presence.  They may arrive via e-mails or downloads.

What impact may viruses and worms have on your business?
A virus or worm will slow down your company’s systems and productivity as you need to dedicate resources to remove it.  Other impacts might be stolen information (yours or your customers’), increased spam, denial of service, or deleted files.

How do you know your computer is infected?
Unfortunately, there is no particular way to identify that your computer has been infected with malicious code. Some infections may completely destroy files and shut down your computer, while others may only subtly affect your computer's normal operations. Be aware of any unusual or unexpected behaviors. If you are running anti-virus software, it may alert you that it has found malicious code on your computer. The anti-virus software may be able to clean the malicious code automatically, but if it can't, you will need to take additional steps.

What can you do if you are infected?

Minimize the damage – Contact your IT department or consultant immediately. The sooner they can investigate and clean your computer, the less damage to your computer and other computers on the network. If you are on a home computer or a laptop, disconnect your computer from the Internet. By removing the Internet connection, you prevent an attacker or virus from being able to access your computer and perform tasks such as locating personal data, manipulating or deleting files, or using your computer to attack other computers.

Remove the malicious code - If you have anti-virus software installed on your computer, update the virus definitions (if possible), and perform a manual scan of your entire system. If you do not have anti-virus software, you can purchase it at a local computer store.  If the software can't locate and remove the infection, you may need to reinstall your operating system, usually with a system restore disk that is often supplied with a new computer. Note that reinstalling or restoring the operating system typically erases all of your files and any additional software that you have installed on your computer.

Resources:
National Cyber Alert System Tip: Recovering from Viruses, Worms, and Trojan Horses