Windows is a great operating system, but it does have problems with security. To keep viruses and hackers off your computer, you need additional security programs.

Why not grab a security system right from the source? Microsoft Security Essentials is Microsoft's free antivirus program.

It has a powerful malware-detection and removal engine that rivals the best paid programs. Plus, it integrates very well with Windows, as you would hope.

The program is lightweight, so it shouldn't bog your system down. Automatic updates keep you protected at all times.

MSE is one of the best free options for small business owners as well. Most free security software is for personal use only, but Microsoft has authorized MSE for use on business computers.

Cost: Free

Link: windows.microsoft.com 

System: Windows XP, Vista and 7