AVG antivirus is provided in a free version for personal, non-business use. It is a basic solution to virus protection and an absolutely essential piece of software if you have no antivirus software installed. There are commercial version of the software available which provide broader protection and are well worth considering.
Ad-Aware from Lavasoft is a useful addition to the antivirus software allowing you to scan your computer for spyware and also remove it if found. Again there are various versions available providing more services at a cost.
ZoneAlarm provide a free version of their firewall that provides a degree of protection to your computer. Firewalls can be problematic to use if you are not an experienced user but if you want to see what a third party firewall can do this is a good place to start.