If possible, you should consider getting a backup virus scanning program like Norton Internet Security, their best deals and offers will also help minimise the costs associated with purchasing the program. Only one program should run constantly, but it can often be a good idea to double check files before you open them.
In addition, both Microsoft and Apple send out regular updates to their various operating systems. There are even updates for Linux OS, though the actual need for such things is generally low. You can often set your updates to run automatically. If you're not sure whether or not this is a good idea, then you should do it.