Computer viruses are a big menace and they can cause a lot of havoc if not tamed in time. I used the word tame because they can easily be tamed with use of anti-virus tools and in-time updating of your computer. Computer viruses can delete or alter files on your computer or across networks, change settings, send/receive data across the internet, let others control your computer or even erase all data in your computer. Hell today's viruses can even update themselves on the Internet to counter anti-virus programs. Viruses can infect other programs and keep spreading over your floppies or compact disks. They are able to do this by residing into the memory of your computer or when you execute a infected application program.
I personally consider trojan horses as the most dangerous types of viruses. Basically a trojan horse is a program that enables someone else to control your computer by copying/editing/deleting your files, peeking into your web cam or by causing unexpected activities on your computer. Typically hackers embed a trojan horse program into a application or software program that can be considered legitimate and upon execution of the application by a computer user, the trojan horse copies itself into the computer memory and runs each time the computer is started. The trojan horse virus accepts commands over the internet by the person who created or spread it; these commands then perform a set of pre-defined tasks such as stealing your files or viewing your web cam. Some popular trojan horse programs which hackers use are SubSeven, Back Orifice and Gupt.
Multipartite viruses affect more than one application and have the capability of replicating themselves. They have the characteristics of both boot sector and file infecting viruses. The first of such kind was known as Ghostball which was created by a person named Fridrik Skulason. These viruses are very hard to remove as they keep on infecting other files and even if they are removed from the main memory, they initiate again upon execution of the infected programs.
Well, I will begin with a formalized definition for Boot Sector Viruses. Boot sector viruses are those viruses which affect the boot sector. Pretty lame definition right" Yeah! I know, I will explain first what exactly a boot sector is. Boot sector is an important part on your hard disk that consists of information on your hard disk partitions and also consists of a Master Boot Record (MBR) that consists of information on your operating system. So, usually if this sector is damaged, your computer won't boot and you wont be able to start your operating system. Boot sector is primarily the first sector on your hard disk. If this sector is infected by a virus, either your system won't start at all or the virus will have the ability to self-replicate by spreading over the network or infecting other files on your computer.
Macro viruses are viruses which exploit the core functionality within your Microsoft Office documents. These viruses have the ability to launch and spread upon execution of Microsoft Office documents. If you are using the latest version of Microsoft Office and have a good anti-virus program, you don't have to worry much about these kinds of viruses. However, if you are using old versions of Microsoft Office, your computer has higher chances of virus infection. These viruses have the ability to alter office documents and even render them useless. These viruses can even have the logic of converting themselves into executables and spreading over network or through other files.
As the name suggests, network viruses are typically viruses which spread over a network - be it a local area network or over the internet. These viruses can be classified into both not so dangerous and dangerous categories. These types of viruses typically are coded for a purpose. The purpose may be alternation or deletion of files, bringing a network down through DOS (Denial of Service) attacks or just spreading themselves until a action performing command is executed over a network. I do remember one such type of virus, it was named Evilbot. Evilbot has a evil history of shutting down giant corporations through carrying out denial of service attacks. Explaining this type of viruses is beyond the scope of this article.
Logic bombs have all the basic characteristics of typical viruses. The thing that differentiates it from the rest is that, logic bombs don't jump upon execution and start damaging your files, instead they wait for the right time to execute themselves. The right time I am talking about may be a exact date or a sequence of events. In short they are programmed to do whatever they are hard coded to do on a specific time or date or it gets triggered upon some events. These kinds of viruses need not necessarily be dangerous, sometimes they are simply used to play pranks. However, this does not diminish their potential of destroying information on your computer. These types of viruses can go undetected for as long as they are actually triggered.
If you are considering for a paid version I would (without blinking my eye) say go for ZoneAlarm Antivirus from ZoneLabs (www.zonealarm.com). ZoneAlarm is a very effective protection software for all your security needs. ZoneAlarm comes in a wide array of versions; if you have the budget don't think much and go buy the full version with all capabilities such as firewall, anti-virus, anti-spyware and email protection. I have been using ZoneAlarm since years and consider this as the best security software; this has helped me keep protected from all those nasty viruses and hackers for long.