why do i need antivirus software

Well people
i have been totally confused
my computer has AVG an antivirus software well last time i downlaoded a file from site
after downlaod during installation the trojan pop up menu do appeared but i think my computer still is infected by virus or trojan then why do i need that antivirus software to give me news
cant it stop it
i was surprised
any way many people say trojan are only info stealer but why is it making my computer freeze infecting system files
please some body make me clear
thanks
by the if i have to use fire wall can u suggest me which really can stop virus even entering in to my computer may be stop virus and whole download it self
which one can i use with win 98 for one computer

Hi saharika.

There are many threats to computer security, viruses and trojans are two, there are also worms, which may use DirectX or various types of scripting, and assorted spyware/ad-ware/keyloggers as well.

An anti-virus program usually uses different ways of detecting viruses, two major ones are signature detection, and detecting code that performs suspicious actions (heuristic detection).

AVG is a decent antivirus, but it may not be the strongest at dealing with trojans. I'd say it is giving you the warning because its heuristics have detected code that may be harmful, but is not a virus it knows how to deal with.

A trojan can draw in other forms of attack like a worm, which may be the cause of system file infection/corruption.

On-line trojan scans can be helpful, some well known ones are:

www.trojanscan.com (fast but may not be most up-to date)

http://www.pandasoftware.com/actives..._principal.htm

http://housecall.trendmicro.com/hous...start_corp.asp

As far as preventing things from getting onto your system, you can use a commercial Internet Security package, or a range of individual protection measures, a number of which can be free.

AdAware, Spybot Search&Destroy and SpywareBlaster are useful tools and their use is recommended.

A firewall is virtually essential, but doesn't stop viruses, it protects against attempts to make unwanted contact with your system, or warns of anything trying to make unauthorised accesses outward from within your computer eg a trojan or worm.

I use Outpost Free myself:

http://www.agnitum.com/download/outpost1.html

It's very light on system resources which is good for Win9x.

Other folk have good results with Zone Alarm and Kerio.

saharika--As Platypus has said, there are many bad things on the internet that can infect your PC. It, unfortunately, is just a fact of life when you use a PC on the Internet these days. It is possible to protect against most of the bad things, but it takes work.
The bad things, by the way, can be anything from minor irritants, like giving you popups, to reporting personal information about you to others, to totally corrupting your PC so that it no longer works.
So that is why you should try to protect your PC as much as possible.
Here is an article which has a comprehensive list of programs you can use to protect your PC. (Platypus has mentioned some already)
http://www.windowsbbs.com/showthread.php?t=31695
Another possible useful article (with links) is
https://netfiles.uiuc.edu/ehowes/www/main.htm
I am not sure if either of these emphasize the need to keep programs like Antivirus, spyware detectors and SpywareBlaster up to date with the latest virus definitions and reference files. If you do not do this, the programs will soon lose their ability to protect.
Also Internet Explorer has many built in Security features, but you have to set them properly to work. This is mentioned near the bottom of the windowsbbs.com article, but easy to miss.
I hope I have not discouraged you. The good news is that many of the programs to protect your PC are free. :)
Feel free to ask more questions.