First up is Kaspersky Lab Internet Security 2013 Software (covering 3 PCs for a year), free, including Free two-day Shipping (after rebate). Newegg.com charges sales tax in California, New Jersey, and Tennessee, by the way.
You might exclaim, "But this is the year-old 2013 version!" However, remember that security companies all use the subscription model (unless, naturally, their product is free). That means that if you have a key or keys for an earlier year's product, you can use that and download the current version from the developer's website. In this case, you would be able to use the current version, Kasperky Internet Security 2014 for a year after you activate your software.
Kaspersky Labs is one of the pre-eminent security firms. Its software is almost always highly rated, and the software -- of course, you mileage will vary -- does not impact the performance of most PCs. The Internet Security package is their middle-tier package. They have a simple antivirus package, Kaspersky Internet Security or KIS (which adds features to target Internet threats), Kaspersky Pure, and Kaspersky Internet Security for Multi-Devices.
In comparison, there is some strange pricing at Amazon.com, with the Kaspersky Anti-Virus (KAV) software being priced at $49.99 while KIS (both 2014 versions and 3-user, by the way) is priced at $38.07. In either case, with this rebate (which admittedly, as a mail-in rebate, is somewhat risky), Newegg.com's pricing is much better.
As a second choice, Newegg.com also has the Panda Internet Security 2014 software package (3 PCs), also free after mail-in-rebate. Normal pricing would be $30. Shipping is again free, with two-day shipping as the option. Denoted with the 2014 term, this is of course Panda's current version, but as noted above, since these packages are sold with a subscription model, there is really no issue with buying last year's version.
Notably, though, while these are both free, by next year, you will be forced to ante up for a new license. It's possible, though, that similar deals may be available at that time, although you would have to reactivate the software with different serial numbers / activation keys.
For those not wanting to bother with jumping through hoops, there are free antivirus software packages that many rely on, such as Avast, Antivir, AVG, and more. These are not trial versions, but omit a few things, such as multiple antivirus definition updates on a single day. These companies generally offer a paid version, as well.