Some stuff on CNET comes with their own 'installer', which can, if you're not careful, install stuff onto your hard drive that you'd probably rather not have. (It's not actually 'dangerous'; it's simply annoying). However, even when that 'installer' route is required to get a program from CNET, you can still get rid of the unwanted stuff simply by selecting 'custom install' and then removing all of the ticks against anything you don't want. So I ner have a problem downloading anything from CNET. (Methyl hates it though!).
However Avast doesn't come with CNET's 'installer', so you're doubly safe with downloading it. Go ahead and install Avast from there.
Once you've installed Avast, make sure that you turn 'Sounds' off. (If you don't, you'll frequently hear an annoying American woman's voice telling you that it's been updated. She can drive you up the wall!). It's dead easy to do so. You simply click on the Avast icon at the bottom right-hand corner of your screen. (It's a curly 'a' inside a mustard-coloured circle. You might need to go via 'Show hidden icons' to see it). Click Settings, then Sounds. Click to remove the tick and then on OK. Job done!