You can download AVG for Windows XP free at http://free.grisoft.com
AVG free is a complete antivirus software package that you can use free of charge as long as you use it for private, non-commercial, single home computer use only.
If you are looking for antivirus software for use in an organization, for commercial purposes or a home business, you can take a look at AVG's commercial version, AVG professional. The professional version of AVG for Windows XP is not free, but for those who want 24/7 technical support included, it's a cheap alternative.
McAfee Internet Security Suite (http://us.mcafee.com) is another option for (home) office virus protection. It offers the additional benefit of anti-spam, anti-spyware and firewall functionality to support your productivity.
There are several reasons why software like AVG for Windows XP shouldn't necessarily perform less than commercial packages :
AVG for Windows XP is not the only anti virus software out there :
Avast Home Edition (http://www.avast.com/eng/download-avast-home.html)
You have to download and install an evaluation version. You can use it for three months, after which you need to register the software. If you don't register, the virus scanner stops working. If you do register, your evaluation version changes into a permanent one and from then on you have full access to updates.
Antivir Personal Edition (http://www.free-av.com)
Free virus protection from Germany. You also have to download and install the software, but you don't need to register. During the updates you will see promotional messages advertising the advantages of paying subscriptions.
Anyone that uses the internet should get decent virus protection.
And even then, you should be careful with doubtful e-mail attachments and websites with pop-ups that ask you to click "YES" or "OK" to continue. Never underestimate the cleverness of virus makers.
Should your computer become infected despite an anti virus software like AVG for Windows XP or even a commercial virus scanner like McAfee, then there's a good chance that the virus attacks your virus scanner. Virus scanners that have been attacked by malicious viruses often don't update anymore or they might even get disabled.
If you are left with a compromised virus scanner, try to do a complete system scan with an online virus scanner. An online virus scanner simply works in your internet browser. When using an online virus scanner, you usually DO have to click "OK" or "YES" somewhere in the process, because the online anti virus program might need to install its code in your browser.
TrendMicro's HouseCall can scan your pc online when your own anti virus software is out of order.
Pc PitStop (http://www.pcpitstop.com/antivirus/default.asp)
Pc PitStop uses the Panda Software virus scanner to scan and clean your computer.
The BitDefender online virus scanner.
Symantec Antivirus (http://security.symantec.com/ssc)
Some viruses silently do their work in the background and copy your files to their own archive. Then the virus deletes the original files from your computer. Next you get to see a message that your files have been encrypted and that you have to send an email to a specific e-mail address. If you send the e-mail, you receive an answer telling you that you need to buy something in a certain online shop to get your files back. These kind of computer blackmail viruses are called ransomware.
Another example is the creation of zombie pc's. Large networks with thousands of home computers get infected with a special type of virus. The virus silently sits on these computers and does nothing. Malicious organizations sell these networks with infected computers to spammers together with an activation code for the viruses. The spammers then use the activation code as a wake-up call for the viruses and abuse the zombie computers to send their spam messages to thousands of internet users.
Computer viruses are big business. We strongly encourage you to install protective software. Free virus solutions such as AVG for Windows XP make it affordable for everyone.