So you have decided to change antivirus software on your computer , here is what you should do.
First make sure you thoroughly remove your current anti virus on your computer.
Your antivirus solution can be un-installed from the Programs section in your Control Panel however some remnants may remain so you need to go to the manufacturers site and look for a removal tool which will remove all remnants of the software.
I have included some links below for popular anti virus solutions.
Norton Removal Tool
McAfee Removal Tool
Kaspersky Removal Tool
AVG Removal Tool
After removal reboot and proceed to install your new anti-virus solution.
The two I recommend are :
The FREE solution : Microsoft Security Essentials - please note , I do not recommend AVG
For a paid solution ESET / NOD32 a powerful antivirus solution that does a great job without slowing your computer down or Norton , Kaspersky, Trend solutions.
Remember to stay " infection free " it is all about - "what you do" as the antivirus solution may not save you and remember to follow my 10 Steps For A Virus Free Computer "