cameccas

Hi why does everytime i download an antivirus program like MC AFEE or NORTON
my computer starts running more slowly ? it even freezes,why? doesn't it has
to run better because it's cleaning all the bad stuff??
I even baught the NORTON 360 ALL-IN-ONE SECURITY and it made my computer run
very ,very slow !!! what program can I get that would make my PC run better
and safe ??