As was said, nothing is 100%. I've run AVAST off and on for years. Only once has anything gotten onto my computer. Then, sandboxing, MBAM, Hitman Pro etc didn't exist. I managed to do a couple of online scans, and one of them caught it.
Now, I have AVAST IS, thanks to winning one of their Christmas giveaways. If I'm going to sites I'm not familiar with, I use free Wondershare Time Freeze as an extra layer of protection. I've also got MBAM and Hitman Pro (both on demand) just in case. These days, even safe sites aren't always safe any more. It just pays to have something else available.