How does an antivirus software find out whether a file is a virus or not?

I opened a USB drive today and my AVG popped up - THREAT DETECTED.
i wonder what is the logic behind the functioning of an antivirus software? how can it scan a pc so quickly and find so many different types of virii?

But how can the system compare every file to such a huge database of virii.
because obviously it has to check every file for every type of virus infection.
i am just too curious....

Every virus has a virus signature, much like a finger print. A signature is a unique string of bits, or the binary pattern and virus scan software use this signature to detect and identify specific viruses. it scan through the files and keep comparing it to the signature files (usually dat file) it has. Once a virus signature has been detected, it will flag it as virus. Some viruses share the same signature.

http://antivirus.about.com/od/whatisavir...

A virus is a program that copies itself to the memory and tries to copy itself to other application that are open.
Viruses can come from
1)Email attachments
2)Floppies(if used previously in a infected computer)
3)Pen drives (if used previously in a infected computer)
4)etc.
Viruses basically destroy the system files in the computer so that you have to buy a new one.
As long as you update AVG regularly then you can be 50% safe. The anti-virus software compares a suspicious file to a large database the database is updated regularly when you run the update in the software.

scanned it

Good question - but it's instant technology at work.

It compares the coding of various files to ones that it has in memory (in the "library"). This is why you need to update your antivirus software so often--it needs to download the identities of new viruses.

Antivirus software typically uses two different techniques to accomplish this:

* Examining (scanning) files to look for known viruses matching definitions in a virus dictionary
* Identifying suspicious behavior from any computer program which might indicate infection. Such analysis may include data captures, port monitoring and other methods.

Most commercial antivirus software uses both of these approaches, with an emphasis on the virus dictionary approach. source http://en.wikipedia.org/wiki/Antivirus
Regards,
G.
http://www.howtoguidehome.com

virus has extensions and values that an anti-virus database has stored in it..

edit: it checks for virus in the system files first den goes to the rootkit den registry and so on.. the more files u have the more time it takes..

thyis is a good question .but there is a simple process behind it.

the antivirus softwares only check the extention i.e it only checks any change in the exten. which is suspecious..

and this process is very fast so the antivirus can manage it.
also we update the program for newer virus definitions only.
after the scanning it simply asks the user to delete the suspected file or repair if possible..

there r various extension of viruses through which the anti virus get to know that it is virus.