If you have had a bad experience yourself with virus infections on your computer you will know the value of preventing computer malware and virus infection through installing appropriate virus protection. Here I give my recommendations for both paid and free antivirus software.
Paid for Antivirus Protection
I recommend Norton Security Deluxe 3.0 - 1 User, 5 Devices and Utilities 12 Month License Card (PC/Mac). According to the independent organisation AV-Test, Norton products are amongst the few which offer the highest protection against malware (spyware, trojans, viruses).
From personal experience I would not recommend McAfee products, which are often bundled with new laptops and PCs. McAfee offers poorer protection than other choices.
Free Antivirus Protection
AVG antivirus is the free product that I would recommend. It does not offer the same level of protection as Norton, but is a good alternative.
You should always be careful, particularly when opening email attachments - if you don't know who sent it, don't open it - and with surfing the internet, following links, etc.