Norton Antivirus. Been using it all my life and never had one problem with it. It gave me a number of warnings too (virus alert-so I know it's working) so thank goodness for that. Some people have a hard time with that software which I don't know why. Make sure during the installation process, close out all programs, disable and remove all exisitng antivirus programs before attempting to install Norton. Everything should be smooth as a baby's butt.
My work uses Panda Titanium which does an ok job, but it's a piece of crap when compared to Norton. They've gone cheap because they whine about not being able to afford licensing.