Regardless of the antivirus program you use, it will impact your computer's performance. Some more some less. Even programs which incur annual subscription fees are not always cost effective when you consider how much they slow down your computer. For many years we've recommended AVG and for the most part still do. There are a number of excellent antivirus programs that are free and do a terrific job.
We would also like to let our user's know about the Microsoft Security Essentials (MSE). This is a complete anti-malware package available from Microsoft itself. So, if you are running windows XP (or newer) and your $50 copy of Norton or McAfee is about to expire, consider uninstalling it and using the MSE software.
MSE does have some system requirements that you should be aware of before cutting loose your old program. If you are running Windows XP operating system, you need have your system up to date with Service Pack 3 (SP3). Newer operating systems such as Vista and Windows 7 are ready to go.
Other requirements are:
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.
200 MB of available hard disk space.
For more information about MSE, or to download it, visit the Microsoft website
If plan to use this antivirus program, be sure to completely uninstall your old program through the program manager found in your control panel.