Prevx1 is indeed a great piece of software. its based on a community database which makes it a lot stronger then most others. also, it does a pretty through job of checking and rechecking after it has cleaned up any malicous items. i give it two thumbs up
This is a personal thing. Some swear by AVG while others, myself included, say Avast is better. Search around for product test results and you will see that sometimes AVG comes out and sometimes it is avast.
as mak said, its a personal choice. i find avast detects virus's as they are come through the wire, where as avg may or may not detect it until you run a scan.