The simple answer is No.
There are plenty of free options out there. Yes, free. Some anti-virus companies actually allow you to download working copies of their software, however there are generally 3 main conditions:
1) The free version of the software obviously has a more limited set of functions as opposed to paid version. For example: some suites may not provide real-time protection in their free version.
2) Some have annoying advertisements and continuously nag you to purchase. Fortunately a few are not as annoying as the rest.
3) Updates are often provided a bit slower than the paid version. What this means is that if you are a paid user you often will receive security updates a bit faster than a free user.
Regardless of the conditions, free Anti-Virus software is a definitely viable alternative to purchasing expensive Anti-Virus software. As a free solution + awareness of what media, websites and software you browse/install on your PC will protect you from 90% of malware and viruses.
That being said I must mention though that the best, most expensive and most courteous users have no guarantee that they will be never affected by some form of virus, malware or trojan during their computing experiences.
Here at ITCoffee all of our consultants and freelancers have been using some or other free Anti-Virus software on their personal machines.
Personally I recommend AVG Free, as I have been using it for more than 8 years, and have since; one minor incident to share (which was more as a result of a bad java update than anything else).
More questions? Shout in the comments.
Links to free Anti-Virus options: