Viruses and malware are a nearly universal aspect of modern computing. Even Macintosh computers are increasingly targeted by them. Anti-virus programs exist for all major computing platforms, though sometimes installation on an already infected machine can be difficult because the virus monitors network connections and prevents a successful download or installation of an anti-virus software package.
"Side-loading" is a term borrowed from mobile computing, and means loading a program through a removable storage device, rather than downloading it off of the Internet. This applies to PCs as well as mobile phones; copy the installer for your anti-virus software onto a USB thumb drive on the uninfected PC, plug the thumb drive into the infected PC and run the installer off of the USB drive. Some malware is designed to infect PCs from a connected USB flash drive, so although this may be the easiest way to load an anti-virus installer on the computer, it's not necessarily the most secure.
Another way to get an anti-virus installer onto an infected PC without putting that PC on the Internet is to use a local area network, or LAN. "Network" doesn't always mean the Internet: lots of home networks have media servers, music servers or shared storage servers. You can download the software onto a network location that your infected computer can access (usually called a file share) and run it off the network. In more structured network environments, such as a small business's IT department, this may be the preferred way to roll out anti-virus updates because the computer can be configured to grab the latest update whenever it connects to the LAN.
File Sharing Services
Dropbox, Google Drive and SkyDrive are all free services that allow you to share parts of your hard drive over the Internet, and they all use different ports and download to different locations, which may keep a virus from recognizing that an anti-virus package has been downloaded. If an extant virus infection prevents you from downloading an anti-virus installer, have a friend download it and put it in your shared folder with one of these services. You'll be able to run the installer from the shared folder, just like you would from a LAN.
Computers with CD-ROM drives are becoming increasingly rare, but many older computers do include one. Saving the anti-virus installer on a CD can let you install it on an infected PC. This is particularly handy when dealing with an infected PC that's in the hands of a less-than-savvy computer user, such as an elderly relative or a young child.
Most new computers sold have some sort of anti-virus already installed, even if it's Microsoft Security Suite. "Most" is not the same as "all." If you're building a machine at home and need to install anti-virus software on it, use one of these techniques to get anti-virus software loaded before taking the computer on the Internet.
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