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1) What are the different ways by which a computer could get infected by virus or nay other malicious programs.

2) What precaution should I take

3) What to do if I get infected by a new kind of Virus which my anti-virus fails to detect and clean

4) What are the chances of me propagating the malicious program without my knowledge

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1) Ask one question only. 2) Possible duplicate: superuser.com/questions/100360/… –  Hello71 Aug 31 '10 at 14:51
    
@ hello 1) i am sorry for asking multiple questions , i thought all other questions are related so i asked in one post 2) the question that your link directs asks how to clean a virus infected computer whereas my question is "how to take precautions" –  subanki Aug 31 '10 at 15:15
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3 Answers

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Q1) There are many ways a computer can get infected with a virus.

Here are some common ones:

Accepting without reading

By far one of the most common ways a computer becomes infected is the user simply accepts what he or she sees on the screen without reading the prompt and/or understand what it's asking.

Opening e-mail attachments

Another very common way people become infected with viruses and other spyware is by opening e-mail attachments, even when from a co-worker, friend, or family member. E-mail addresses can be easily faked and even when not faked your acquaintance may unsuspectingly be forwarding you an infected file.

Not running the latest updates

Many of the updates, especially those associated with Microsoft Windows and other operating systems and programs, are security updates. Running a program or operating system that is not up-to-date with the latest updates can be a big security risk and can be a way your computer becomes infected.

In addition to running operating system updates, the plugins associated with your browser can often contain security vulnerabilities. Make sure you have the latest versions of plugins.

Pirating software, music and/or movies

If you or someone on your computer is participating in underground places on the Internet where you're downloading copyrighted music, movies, software, etc. for free, often many of the files can contain viruses, spyware and/or malicious software.

No anti-virus spyware scanner

If you're running a computer with Microsoft Windows it's highly recommended you have some form of anti-virus and spyware protection on that computer to help clean it from any infections currently on the computer and to help prevent any future infections.

Finally, downloading any other software from the Internet can also contain viruses and other malware. When downloading any type of software (programs, utilities, games, updates, demos, etc.), make sure you're downloading the software from a reliable source and while installing it you're reading all prompts about what the program is putting on your computer.

Caution: Some websites have scripts that automatically downloads into your computer as a virus.

Originally from ComputerHope with a few edits.

Q2) Always update your anti-virus engine and virus definitions.

Also, be careful in what you're downloading, especially pirated software as many of them contain malware.

Download software from trusted sites and not from random places.

Do regular malware scans.

And be careful. Anything could happen, even at trusted sites, so always be on the lookout.

3) Anti-virus software usually detect malware by examining files for known virus code/signatures in a virus code or looking for any suspicious behavior that may be linked to malware. Many malware often contain similar codes or signatures.

In a case that this happens, try updating your anti-virus first. Then, do a virus scan and remove malicious items. Report the malware online. IF none of these works, as a last resort, make sure you have a non-malware infected backup and reinstall your OS.

4) I'm too lazy to answer this question. Ask someone else to do it.

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You should probably put the reference that you got that from: computerhope –  KronoS Aug 31 '10 at 15:29
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anwser to question 2: Use antivirus with residental scanner and use av special options like loading into ram before explorer etc (Avira Antivirus has that option to load into ram before explorer process i believe). Use heuristics and use all scaning options in real time (it can be pain in the ##s but that is the best that modern AV's can do about viruses in real time).

Create backups very often. By backup i menat files that you created and that will be lost forever if you dont backup them to read only media (dvd or something even larger) and read/write media (usb mem, external hard disk etc.). Scan your backups at your friends place with their AV's.

Dont use autorun on inserted media. Notice that many viruses are able to copy themselves into your pc by using autorun on your pc. Autorun is not crucial for pc anyway since you know when you inserted any new media and you will open your file manager on your own will when you need something from inserted media. Check this out: https://encrypted.google.com/search?q=disable+autorun+in+windows

Use executable fillter on your machine. It will prevent unwanted apps from running. If you are using WindowsXP check this: http://www.beyondlogic.org/solutions/trust-no-exe/trust-no-exe.htm

CLean your browser cache often and remeber that you have to visit web pages very carefully if you want to your data to be safe. ALso remeber that there is no best way to protect your pc. Even if you do all that i mentioned there is a chance that your pc will be infected.

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Drive-by-downloads

Downloading anything on the internet is a potential risk, period, although in practice it's fairly small. A trusted site can get taken over by a cross-site scripting attack, host a malicious ad, etc.

I don't mean "download" where you explicitly save something to your computer. Even viewing a website requires you to download it.

Flash and Silverlight (and anything else EXECUTED, even in a sandbox) are more risky than than just media, but a bug in the media loading routines can cause problems.

This is relatively rare but every once in a while someone will discover a previously unknown vulnerability (a zero-day) and use it against you.

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