Steam – Why Do Some Friends Have Yellow Names and How to Get It?

Not all games available on Steam work with newer PCs.

While some old games "just work" on newer OSs, many other games have compatibility issues which make them buggy or unplayable when you don't have the "required" OS. For example, the Space Quest collection has trouble running on Vista/7.

I would say that if a game doesn't state your OS specifically, you should probably check for known compatibility issues before buying it, especially since you mention you're using Win 8.

An alternative would be Good Old Games, which specifically mention a game's compatibility with newer OSs, if you're unsure whether the Steam version is compatible or not, I'd go with them.

As of this date, you cannot turn off the notifications from the groups you have subscribed. You cannot turn off the events notification as well.

In near future steam might implement this feature through updates because many people have reported this to steam including me. But for now we are out of luck.