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A lot of computers that come preloaded with Windows may also have a preloaded anti-virus program like Norton or McAfee. Thing is though, that these programs are trial versions that expire after a while and then prompt you to pay for a full usage license. Typically the update functions cease until you buy the product. So you have the choice to renew with them or use a different product.
It is kinda sucky though; when you buy something it should be reasonable to assume that what you have and all within it is yours and there should be no need for further payment to keep it all going. But, maybe that's a caveat emptor thing and all...

Anyways, here's the buggy part. If you chose not to continue with that product, it can sometimes(but not necessarily always) be difficult to uninstall that product before installing a different one. McAfee and Norton are both known to be like that.
The "usual" way to uninstall software is through 'add/remove programs' in control panel. That doesn't always work. The program may or may not uninstall, uninstall only partially, or sometimes not even show up in the list as a removable item. In that regard, it might be accurate to classify these products as "Cling-Ware", that is- stuff that just won't go away; ironically, they are now similar in function to the stuff they're supposed to protect you from. Not that I'm slamming these products' effectiveness; they do what they do well enough. It's just that when it's time to go, they sometimes make a hard time of it.

So, current case in point then; my McAfee expired after 3 months. I chose to discontinue McAfee and use Avast(free home edition) instead. Due to "challenges"(details of which were almost certainly repressed by a psychological self defense mechanism) during the intended uninstall process, I eventually ended up using something called the 'McAfee Consumer Products Removal' tool. Naturally I can't predict anyone else's results but it works ok for me. And my system seems to feel crisper and lighter too. Subsequent searches show no evidence of any active McAfee components remaining.

MCPR.exe download links
http://www.pcworld.com/downloads/file/fid,73330-order,4-page,1/description.html
http://www.filecluster.com/downloads/McAfee-Consumer-Product-Removal-Tool.html
http://www.softpedia.com/get/Tweak/Uninstallers/McAfee-Consumer-Product-Removal-Tool.shtml