Computer Security Applications - Purchased or Free Software
There are some free anti-spyware and anti-virus software products you can download, which do a reasonable job. Although if your wanting a reliable and solid Internet security software from a reputable source I would recommend you take a look at my Internet security guide and anti-virus guide.
With any free internet security software, although some do a good job in detecting spyware, viruses and other threats, it is not their responsibility to keep on top of latest threats, as clearly you have not purchased the product. On top of this, free internet security software products are never usually a reputable vendor and so you can't be too sure in how good they are in keeping your system safe. This is why you should invest in an All-In-One Suite Internet Security Suite from a reputable source where you will have the peace of mind that you have an internet security software product keeping your system safe from current and new viruses as well as other threats.
Saying that, there are some good free internet security software products which do let you use their signature database as a freebie, although with the free internet security software having very limited functionality, and missing out on some of the other essential security features if you had a purchased the full version. For example ZoneAlarm and Comodo do very good firewalls.
Some of the smaller vendors are willing to give away a free service to help them build their brand for long term success. AVG is a perfect example. They offer users free internet security software in their downloadable anti-virus software, in which you can manually update and scan for viruses. This will in turn make the AVG a more popular brand and can then use this advantage by offering their full anti-virus software as a purchase. With a full anti-virus package from AVG will also offer spyware detection, scheduled downloads and updates, e-mail scanning, etc. AVG have actually made a very good name for themselves using the above strategy.
So in a few sentences - free internet security software does not provide complete protection, and is very limited to a what it can do. The main objective to vendors providing free internet security software to end users is to eventually get them to purchase their full product version, which would offer complete protection.
By all means, if this is all you require in a free internet security software then so be it. However you may be leaving yourselves exposed in other areas.
Also see Which anti-virus software guide.