If you are a home user or a small business with up to 10 users, you may want to look at using the free Microsoft Security Essentials antivirus found at http://www.microsoft.com/security_essentials. It is free. You must be running genuine Microsoft products. You probably want to take a look at the user agreement here also. It basically says the same as all others. You agree to the terms, not to misuse or make money off their free product. If your business has more than 10 users, you will have to look for a paid product.
Before we get started, remember you should always backup prior to any changes to your system. This is in additional to the normal everyday backups that you do. Make a backup now, before you start doing anything else. If you want to know more about backups read more at How Important Is Your Data.
If you are looking for a quick way to backup your data before you continue, visit Idrive and sign up free for online backups.
Now that you have a good feeling about your backup situation, you will need to download Microsoft Security Essentials (MSE) from the Microsoft site at http://www.microsoft.com/security_essentials. Save it to your desktop so it will be easy to find later.
Perhaps before your are completely finished with the MSE download you could start making a list of the programs that you will need to uninstalling before you can finish installing MSE. First you will want to uninstall any antivirus that may already be installed. Having two antivirus running real time scans together can cause a real mess and slow your system to a crawl. Second you will want to uninstall any firewall or malware product that is installed. Basically anything that it appears MSE will replace needs to be uninstalled before you install MSE.
Now you can start uninstalling your antivirus and malware software. If you get to the point that you must restart your computer, make sure that the download for MSE has completed and you know where to find it. Then restart your computer and finish any needed uninstall instructions. For more information on uninstalling your unneeded antivirus software see Uninstall Unwanted Antivirus.
After all your previous antivirus and malware software has been removed, you are ready to install MSE. The install is fairly straight forward.
1. Run the setup that you downloaded and saved on your desktop earlier
2. Click Next at the Welcome Screen
3. Read the License Agreement, Check the Agree Box, and Click Validate
4. Click Install
5. Click Finish (let it scan the first time)