Avast.

This box came with Norton, and a year's subscription. I had been using AVG on the previous one for a couple of years without either an operating problem or an viral intrusion.

It wasn't long before it began hanging during shutdown, caused by Norton AV background programs. So I disabled Norton and went back to AVG -- voila, no more problems.

Switching to Avast was driven as much by aesthetic as by any other factor: AVG's GUI is decidely old-school; Avast has a number of interesting skins available (including one based on ST:NG's computer panels, complete with sound effects). Avast's virus database updates are a lot less kludgy, too.

I still have the rest of Norton Systemworks (2003 version), and I use it regularly, especially SpeedDisk. That's a habit (and a brand loyalty) that goes back to the DOS days of Norton Commander.