how does virus/spyware/trojan detection software work?

i'm just curious what these programs look for to i dentify a virus, trojan, spyware, etc. i've heard that viruses are identified by strains or something but i really dont know what that is in terms of a computer virus. i guess trojan and spyware scanners have to scan differently, but i'd like to find out how they work. also, how does av handle the virus once it's been found without deleting the file that it's attached to? that kind of stuff. if somebody could point me towards a good tut about this stuff, id appreciate it, i couldnt find anything good on google(lots of stuff about biological viruses and very vague articles written by individual av companies).

Hi !

If you want to know how stuff works...google example, which I just did..."how does a virus program work"...and you will get this...


http://www.bcs.org/review04/articles...rity/virus.htm

or..google..."how can I make an anti-spyware program" which should still answer your question...

Spyware Warrior » How To Make a Good Anti-Spyware Program - or ...
... malware industry but who are looking to make a quick ... vendors who decided to buy an anti-spyware app on ... of the kinds of information his program was neglecting ...
netrn.net/spywareblog/archives/2004/ 12/03/how-to-make-a-good-anti-spyware-program-or-not/ - 40k - Cached - Similar page

Hope it helps...

Eg

what i dont get is how does heuristic scanning know that an executable can do something like format my hard drive, can it read an executable and know that "0110011010101100=hard drive format"? i guess they use this kind of scan is what they use for spyware and trojans too, right? btw, thanx for the link egaladeist :) i was just searching stuff like "+how +"virus detection" +works" and getting nothing good