If you're buying a computer, the salesperson is likely to ask if you'd also like to buy some anti-virus software. And even if they don't, your PC could well have anti-virus software pre-installed, available to use for a free trial period. With so many computer nasties around it makes sense to protect your computer. But should you pay for this protection? Free software is widely available, but would you be compromising security to save money?

Not surprisingly, some vendors of commercial anti-virus software think so. Speaking on the Tech Blorge website, David Hall, a product manager at Symantec (whose Norton AntiVirus 2010 costs £39.99), warned: "If you are only relying on free anti-virus in this modern age, you are not getting the protection you need to be able to stay clean and have a reasonable chance of avoiding identity theft … free anti-virus is not enough: you need in-depth layered technologies, which only come from the more mature paid suites."

But other anti-virus software vendors take a contrary view, including AVG, which offers both free and paid-for products. Tony Anscombe, AVG's ambassador of free products, says its Anti-virus Free Edition has been downloaded more than 100m times. So why opt for the paid-for version? "If you are using a computer for email and some light [web] surfing, then you probably don't need some of the features found on the paid version," he says. "But if you use online banking or have your home finances on your PC, you'll probably want more features and something that is more integrated."

Both free and paid-for anti-virus products have the same basic software components, and offer the same level of core protection. In other words, the free version is not a weaker version of the commercial offering. But paid-for versions do offer additional features, which can include protection against potentially harmful websites, email protection, anti-spam, a firewall and phishing protection. There can be other bonuses too, such as automatic updates and free technical support.

Most of these features are useful but not essential, or can be found for free.

What you need, what's on offer and where to find it

Most computer protection programs are designed for Windows PCs, because the vast majority of malware is aimed at this platform rather than Apple and Linux computers.

Anti-virus software The Security Essentials package from Microsoft is a free anti-virus package for Windows PCs. Avast! offer free anti-virus package. You can also upgrade to Avast! Pro (£34.95, all prices stated are for one year's protection) which includes website protection, and Avast! Internet Security (£54.95), which also throws in a firewall, anti-spam and a licence for three computers. AVG Free Edition can be upgraded to a full version which includes email protection, anti-spam and technical support for £21.60. AVG Internet Security (£30.39) adds more extras including a firewall. Avira's Antivir Personal Free also includes free support, while the premium version (€19.95) has phishing protection and email protection. Antivir Security Suite (€39.95) also offers anti-spam, firewall protection and a parental filter.

Firewall A firewall is like a protective filter, and is designed to stop unauthorised traffic between your PC and the internet – for example, some spyware can send personal information from your computer to a hacker's website. Windows PCs come with a built-in firewall, which can be switched on via the computer's control panel (if you're not sure how to do this, check your PC's instruction manual). If you use a router for internet access, it may also have a firewall installed – the instruction manual will tell you how to activate it. Free firewalls are available from Comodo, Outpost and Zone Alarm.

More free malware protection software The Windows Malicious Software Removal Tool from Microsoft's scans your Windows PC for malware and removes it. Spybot Search and Destroy also checks for and removes malware. Malwarebytes performs a similar function (there is a paid-for version, too).

Top 10 safety tips

1. Use the latest version of your web browser.

2. Make sure your Windows security updates are up-to-date – the best way is to switch on the automatic update function.

3. Keep your software up-to-date – many updates include security fixes.

4. If you have online banking your bank may offer free anti-virus software.

5. Make sure you only download software from the vendor's website or a reputable website like download.com.

6. Most free software doesn't include technical support, but there are lots of support forums online.

7. Make sure your computer protection software is regularly updated.

8. Scan your computer regularly for malware, at least once a week.

9. Your PC can also be infected by malware hidden on plug-in devices like memory sticks, so scan them too.

10. Don't click on emailed weblinks to banks or other commercial companies – type the company web address into the browser window.

What the jargon means

Adware This displays annoying adverts on your computer, such as pop-ups.

Malware A general term for malicious software, which can harm your computer.

Phishing Criminals attempt to obtain personal information, such as passwords or credit card details, usually by directing victims to fake websites.

Rogueware Software designed to force you to pay for fake anti-virus software, by falsely stating that your computer has multiple infections.

Rootkit Malware that hides deep inside your computer; it can be hard for antivirus software to detect.

Spam Junk email that can clog up your mail inbox.

Spyware Collects personal information from your computer.

Trojan horse Harmless-looking software that can give hackers access to your computer.

Virus A rogue program that infects computers.

Worm A program designed to automatically infect lots of computers.