How Viruses Infect
Are you Infected?
How to Clean

Virus FAQ
 

Q: What is a Computer Virus?

A: "A program that can 'infect' other programs by modifying them to include a possibly evolved copy of itself.”  Viruses can infect program files, such as .com and .exe files, and certain data files like Word and Excel documents that rely on macros.  Viruses can also infect files such as .pdf documents and .jpg pictures, but the code won't be executed by standard programs making them harmless in most cases.

Q: What is a Worm?

A: Computer Worms are programs that copy themselves from system to system.  They generally do not infect .exe and .com files.  Instead they make new files containing the virus code using either preset names or random names up to 8 chars long.  Worms usually replicate over networks, e-mail, and the internet.

Q: What is a Trojan?

A: Trojans are programs that do malicious tasks that the user didn't intend.  Most trojans open a computer to hackers, either by stealing passwords, downloading other viruses to infect the machine, opening up remote control programs to the internet, or turning off security settings to make the machine more vulnerable

While purists draw a firm distinction between viruses, worms, and Trojans, others argue that it is merely a matter of semantics and give the virus moniker to all viruses, worms, and Trojans. The term malware, a.k.a. malicious software, can most easily be used to describe viruses, worms and Trojans while satisfying both arguments. 
Since they all require the same methods to get rid of them and all are equally harmful, I usually don't worry what kind of malware a computer is infected with and just call them all viruses.

How to get infected

Downloading files off the internet - Most files from reputable companies are safe, but even the big companies can get infected by viruses and pass those along to customers downloading files.  Personal web pages can have links to files with viruses.  Sites for downloading pirated programs and cracks are espeacially likely to have trojans usually on purpose to gain control of peoples machines.
E-mail attachments - Files sent by people you know may contain viruses that they are unaware of.  Also, some E-mail worms will send e-mails from an infected computer without that person even knowing.  Always make sure that you know what an attachment is before opening it, and virus scan it even if the other person says it is clean.
Files sent over MSN, Yahoo, AOL and other messenger programs - There are a few viruses that can use chat programs to send files, usually with text like "Check out this cool screensaver" in the hopes that the recipient will open the virus file and infect their machine.  Ask questions about any files recieved to figure out if it is a real person you are talking to.
P2P programs like Limewire, Bearshare, Shareaza, Emule and others - Downloading music and movies is very popular, and a good way for people to spread viruses.  Some people and viruses will choose names of popular songs or programs to try and trick people into downloading and running them.  Most virus files are tiny, about 3 KB while most songs or programs are 4,000 KB or larger.  Care needs to be taken to understand which files can and can't contain viruses.
Using a floppy disk, CD, or thumb drive with infected files on it - Standard antivirus will usually catch any viruses when they are copied to the computer or run from disk, as long as the virus definitions are updated on your computer.

Are you infected?
Viruses can make computers run slow.  They can cause slow internet or cause the computer to keep trying to connect to the internet.  Popups and strange messages can be signs of viruses and/or spyware.  Messages from friends about E-mails you didn't send.  Corrupt documents or picture files.  Many unknown processes running (Ctrl+Alt+Del -> Processes tab). 

How to Clean