Software that suits your needs by following the factors to look before making any purchase: 

• The product is certified by agency.
• Scanning options - Real-time scanning, manual scanning, scheduled scanning.
• Scanning Effectiveness – ability to spot, block and remove the threats.
• You should be able to view the Reports and Activity Log.
• Updates - 24x7x365, Auto Live update, frequency of updates.
• Ease of Use - customization.
• System Efficiency should not reduce.
• Handling Files - Option to automatically clean or Quarantine infected files.
• Product Support - Phone / E-mail / online Help documentation / Live chat.
• Other Protection - Scripting protection, POP3 email protection, IM protection, Registry protection, P2P protection, ActiveX.
• System requirement
• Price

Hope you could have got an idea about the factors to look before you take a decision to purchase Antivirus software by now. The listed Antivirus software’s may not meet all the factors completely but they can protect you up to some extent and rest is in your hand. Do make a comparison chart of all the Antivirus software’s listed. The reason for listing the factors to look is that people should have knowledge about what is Antivirus / Internet security software’s and in what ways they are going to help the online user and the kind of protection they get.