I hate Norton but use Norton Internet Security on all of our (six) PCs because it works and they have helped when we've hit snags. But it does slow your machine down and older versions used to be a bit 'flaky', but touch wood it's been about 4-years since I had a problem. At £22 from Amazon for a 3-PC license (per year) isn't too expensive - I watch for even lower prices just as the old version is morphing into the new, Norton always updates to the newest version anyway. I have 2010 version license but Norton updated it to 2012 FOC.
Regarding scans, new viruses might sneak by your loaded virus definitions, so when the newest definitions are downloaded it's a good idea for the virus scanner just to check if that's happened.
I understand when a virus hits your machine you really regret not having protection software.