Your top requirement for computer security is to know what you have and how to use the programs. If you have a good antivirus (like a watch dog) and don't update (feed) it, then it becomes a waste. If you have a firewall (like a strong gate) but leave the keys at the doorway, it will do what you expect it to do.
If you are not comfortable tweaking settings, I is better to go for Security Essentials.
If you like fiddling with settings, go for Avast antivirus and ZoneAlarm firewall. You can adjust the levels of security as you wish in these. Both are free programs.
Other things to be aware and be alert to, even though these are not antivirus programs as such:
--- You should update your operating system and other program files regularly.
--- Make sure Adobe Flash player gets updated even automatically.
--- Uninstall Java from your system completely, you won't miss it. If perchance you have to use it for some websites, then install it and keep the "Enable Java content in the browser" unchecked. When you need it, put the check mark. When done, disable Java.
If you have time, then you may use the free trials of several full computer security programs for 30 days. Before the 30 days expire, uninstall the existing one and load another company's antivirus. You could probably run a whole year with free trails from 12 brands, and then choose what you felt was your best. BitDefender, Kaspersky, ESET, Norton, McAfee, Panda, Sophos, FSecure, TrendMicro, BullGuard, GData, Lavasoft, WebRoot etc. are some top names.
Be aware that none of these in any combination is 100% effective. Do what you can and pray if you believe!