If you are using a Windows based computer, a good anti-virus software is a must and should. [It is a different story with Linux based systems though ] If you want your computer to be safe and secure, a commercial subscription based anti-virus software is a best bet. However, there are certain "free" anti-virus software too. If you are low on budget, I would recommend downloading one of these:

An important note: NEVER, ever use a pirated/cracked copy of anti-virus software. Many a time, cracked software contain malicious programs that can steal your personal information. Besides, it is illegal to use pirated software. Use a LEGAL free anti-virus software instead!

• Avira AntiVir PersonalEdition Classic: This is one of the best free anti-virus software available. It detects almost all the known viruses and daily updates are available. On the negative side, you will see an advertisement whenever you update, prompting you to upgrade to the commercial version. This is the anti-virus software I am using on my computer right now.
• AVG Antivirus Free Edition: This is another recommended download. AVG is very light on system resources and unlike others it won’t slow the system down. However, on the negative side, the engine is not effective against all viruses and won’t detect many trojans and spyware. There is a commercial version on AVG that is effective against these.
• Avast Home Edition: This anti-virus is another classic! With Avast!, you can expect 100% detection of In-the-Wild viruses (viruses already spreading between users) and excellent detection of Trojan horses. The memory requirements are also reasonable. In fact, it runs very fast and won’t bug your system at all. The home edition is for use at home only and institutions (even non-profit institutions) shouldn’t use it. There is a commercial version available for use by organizations. The software works for 60 days in demo mode. However, you can unlock the software for free by registering your copy of the software.
• ThreatFire: ThreatFire Antivirus is very light on system resources. If you have a computer with low RAM, I would recommend this as it doesn’t slow down the computer even a bit and at the same time, it won’t affect the system startup time. They also offer a paid version which costs $14.95/year.
• ClamWin: ClamWin Free Antivirus is based on Clam AntiVirus engine and uses GNU General Public License by the Free Software Foundation, and is free (as in freedom) software. It is an open source project. Though, it features a high detection rate for viruses and spyware, it lacks a real-time scanner. In order to detect a virus, you have to manually scan the file. Since this is an open source project with continuous improvement, we can expect a real-time scanner in the future editions.

If you really do not want to spend money on anti-virus software, there is another option. Most driver CDs that ship with motherboards have an anti-virus software with about 6 months or more of free updates. The CD which came with my motherboard had Norton Internet Security Suite 2005 with 6 months of free updates. Though not a permanent solution it can still give you 6 months of time to think about buying a subscription.