There are two types of security software. There are free antivirus programs and there are paid ones. You pay an annual fee for updates. Now there is a third option - pay-as-you-go-protection.
Panda antivirus software is now available for £3.99 here in the UK (I don't know if the deal is available elsewhere).
This is an interesting development for several reasons. One is that it covers an unlimited number of devices for what works out at just under £48 a year. That means computers, phones and tablets. It could be useful if you have a family with lots of devices.
Spreading the cost over 12 payments is much more affordable than one annual payment, so this is a bonus.