Two basic precautions can assist in keeping your computer virus free - install and use software from trusted vendors and refrain from opening unexpected email attachments.Nearly all email attachments are possible carriers of computer viruses. Some of the most well known viruses were distributed by accessing address books of infected computers and sending email (with attachments) to every name in the address book. Antivirus software is obtainable in both commercial and freeware versions with an assortment of features and prices generally from free to less than £40. The majority of antivirus software works by examining files as they turn up at the computer and by scanning later at configurable pre-set times. The programs work by looking at each email attachment and downloaded file looking for virus 'signatures'. If a virus is discovered the user will be alerted and infected programs and attachments can be cleansed of the virus or, if not possible, infected files can be deleted or put into 'quarantine' where they can't be run.