The only version I will recommend for free is AVG. The free version will scan for viruses and spyware. You can also get support for this free version from their forums. If you ever decide to upgrade, that option is also available to you. Honestly, the only down side to using this free version is finding it on their website.
Now just because it's free doesn't mean that it isn't cheap or not very effective, it is a very good scanner, in fact, I believe it to be one of the best ones out there. Of course it has one feature missing which is really important to me, which is root kit protection, they save this as a way of enticing you to buy. At least, the price is right.
TIP! Remember to turn on automatic updates to keep your virus scanner up to date. An out of date virus scanner is just like not having one.
For the short term, go with the free version and see if you like the program. If after a year, you enjoy it, get a paid version to enjoy all of the features. Plus you get better support.
Sometimes, your internet service provider will include an antivirus spyware protection in the cost of your monthly fee. You should also contact them to see if this is something that they offer. If it is, you will always have a current virus protection as long as you use the same internet service provider.
After you have your AVG installed, make sure you let it do a full scan if prompted. It is the initial part of the setup process to make sure you computer is currently in a safe state. This is typically something I'll let the computer do before I go to sleep so it doesn't effect my productivity.