Good protection can be had for free.

Microsoft Security Essentials. Microsoft's new antimalware solution, Microsoft Security Essentials, is now available for Windows XP 32-bit with SP2 or SP3, Windows Vista 32-bit, and Windows 7 32-bit. I have found it an unobtrusive app with a clean interface that protected us in the dark corners of the Internet. MSE has one of the simplest and clearest GUIs I have seen for an antimalware solution. Considering the simplicity of the product, it's still very customizable. Power users enjoy having tons of features to tweak, and we think they'll be satisfied with the settings that can be changed in MSE. It doesn't offer as many options as some other security suites, but given its features, the settings that can be adjusted are exactly what one would expect.

System Requirements:
Operating System: Genuine Windows XP (Service Pack 2 or Service Pack 3); Windows Vista (Gold, Service Pack 1, or Service Pack 2); Windows 7

• For Windows XP, a PC with a CPU clock speed of 500 MHz or higher, and 256 MB RAM or higher.
• For Windows Vista and Windows 7, a PC with a CPU clock speed of 1.0 GHz or higher, and 1 GB RAM or higher.
• VGA display of 800 × 600 or higher.
• 140 MB of available hard disk space.
• An Internet connection is required for installation and to download the latest virus and spyware definitions for Microsoft Security Essentials.
• Internet Browser:
  • Windows Internet Explorer 6.0 or later.
  • Mozilla Firefox 2.0 or later.
• Microsoft Security Essentials also supports Windows XP Mode in Windows 7.

Windows Defender for XP, Vista and Win 7 included by default.
Windows Defender is a free program that helps you stay productive by protecting your computer against pop-ups, slow performance and security threats caused by spyware and other potentially unwanted software.