Microsoft is offering free Anti virus software (called Microsoft security essentials) for Windows XP, Vista and Windows 7 operating systems. There is no trial period or any other condition, it’s completely free. You can download it from the below Microsoft portal. The only thing you need to have is genuine Windows OS running on your computer.
On Windows XP SP2, it requires to have Forefront Client Security Filter manager QFE package installed which can be downloaded from the location :. http://support.microsoft.com/kb/914882?mkt=en-us. Installation of this package does not ask for reboot. XP SP3 already has this, so no need to install this package.
If you are already having a anti virus software running on your computer, then you may not need this. Or you can uninstall the other one and then install Microsoft security essentials package. Having multiple anti virus software would definitely impact the performance of your computer.