The first thing you must check is the specifications required to install the software. This is because an older version of the software, though cheaper, may not work with your system, especially if you have bought it recently. Though most companies release newer versions every year, you must nevertheless check the specifications to avoid compatibility issues. For instance, if you have a PC with Windows 7, the software versions 2008 or even 2009 may not work with the OS.

The next thing you must check is the type of virus database update available. Most paid versions provide live updates via the internet. In case you don’t have continuous access to internet you can also download the updates and install it on your system. Make sure both these options are available with the software you get. You must also check how often the virus database is updated. With newer viruses being released almost every day, one cannot be too careful. Some software update their database everyday, while other provide weekly updates. This entirely depends on your requirement and sensitivity of data on your PC. For instance, in case you use your system for business purposes, it is best to get software that provides updates on daily basis.

Apart from this, you must check if the software can be scheduled to scan the system and if idle scan facility is available. More often a virus sits on your system causing damage which may not be very evident, where you come to know of it only when your system has crashed. Idle scan ensures that your system is scanned for virus even when it is not in use.

The last thing you must check is ease of upgrading and compatibility with common OS in use. If you have bought the latest version, chances are the next upgrade will be available a year later. However you must check if the version you have bought works on different platforms and is flexible enough to work on all common OS.

In the virus database method every file coming from another source such as portable storage device or the internet is checked against a database. In case it is found on the database, you are alerted. The software takes action on the file, these including removing the file from the system, putting in quarantine or try to repair the file. The first attempt being trying to repair the file in case it is detected to be infected rather that being the virus itself. In case it cannot be repaired it is quarantined so it cannot duplicate itself or infect similar files. A file is completely deleted if it has been established as a virus beyond any doubt. This method requires regular updates of the database, so that the PC is safe from latest threats. The main drawback of the system is that it is generally not able to detect a virus which has modified itself, thus still wandering about in your system.

In suspicious behavior method the software monitors the functioning of the computer and notes abnormal behavior. For instance, in case a program attempts to change an executable file, the process is noted and you are given an alert, along with actions to be taken. This is a better method compared to virus database, as it alerts the user of a new virus, much before it is updated in the dictionary. However, the drawback is that, even small changes such as normal software upgrades are detected as suspicious behavior, causing unnecessary alerts. This leads to the user ignoring the alerts after some time, thus relying more on the virus database method.

Another method of detecting virus similar to suspicious behavior is the file emulation method. In this method the suspected file is run in a virtual operating system, prompting it to work the way it would when executed in normal OS. In case it shows some abnormal behavior such as modifying code or trying to duplicate itself, it is deleted or quarantined.

A freeware is free for download and distribute. Often the software is owned by an individual and not by any company or organization. The source code of the software is not revealed, however it can be used by one and all. These are generally trial software created by newbies or professionals trying their hands on something new. Such software may be experimental, where its success in performing the task it is supposed to perform is not guaranteed by the creator. In most cases the software are good and occupy very less space. Common freeware software include music players, small games, photo viewers etc.

A shareware is similar to freeware in that it is free for download. However, it comes with a trial period, where you must buy the software in case you want to use it further. Not all shareware are available as trial, some are full versions however some features of the software are disabled, and are made available only when you buy the software. Shareware too can be distributed freely, and source code is not available to the user. Thus, shareware is more of a sample program, where full version is made available once you pay for it.

Open source software sound very similar to freeware. The difference is that the source code of the software is available to users and software developers. The software may or may not be associated with a company; however it comes with support, documentations as well as services, almost like paid software. Such software help developers collaborate with each other and make improvements and enhance features of the software. This also helps normal users, as they get upgrades of free software, without purchasing it. Whether you download these files for personal use or for programming purposes, it is best to check the terms and conditions given by the owner.