I don't understand many of these comments.
There are two good free anti-virus programmes out there.
These offer free annual renewable licenses for home use.
For home and very small business use they are good, provided the computer in question runs Windows XP or Windows Vista and one has a broadband cable/adsl/Vodafone dongle type internet connection and connects regularly to the internet.
All anti-virus companies have teams working 24/7 and are usually onto writing protection updates very quickly. That is why anti-virus programmes are always wanting to update. So when you connect to the internet let the anti-virus programme update fully first before opening email programmes like outlook express.
However, and this is a big however, there are some nasties that are not called viruses but trojans. These tend to infect machines through downloads or clicking on links in badly protected browsers such as old versions of Internet Explorer.
To improve security Windows XP or Vista, one should always make sure that in addition to a good anti-virus programme, one should have the latest Browser installed, (e.g. Internet Explorer 7 or 8, Firefox 3 or Opera 9.6) and also Windows Defender which is a free anti-trojan programme from Microsoft. Make sure all the settings are set to block pop-ups etc.
Also make sure that you firewall is correctly configured.
Also, try to move away from using email programmes such as Outlook Express. If you have a gmail account and broadband, you have practically as much flexibility as the aforementioned programmes. You access the account through your browser and best of all Gmail usually catches the bulk of nasties in emails on google's computers before they get to your machine.
Don't download stuff you don't know where it came from. (e.g. Bittorrent etc.) Don't click on links in webpages you don't trust. Don't use other peoples cds/dvds/digital camera chips without scanning them with your virus progamme first.
Apple and Linux users are generally exempt from the above comments, except as regards firewalls.
I have about 10 machines operating with the above configuration and there is rarely a problem with them.
....The operators also know that they will incur my wrath if there is a hint of misbehaving ::))))))