It's called a "mod" - presumably, since I'm not familiar with the thread you're referring to. Which is a long Paradox tradition, forum members improving the game for free.
Placed in the mod folder, they can add new challenges or features, depending on what they are designed to do; sometimes, they are quite poor at achieving their aims, and make the game an unplayable wreck (Sorry people who tried version .005 ^_^0 ...).
That said, it's always done at your own risk, which is why you should never use a mod outside of the mod folder, so that your original files are safe; at the very least, you should back them up first.
The reason they are hosted at filefront is because Paradox doesn't host the files themselves; it would be a huge investment of bandwidth and space, given the extent of some mods.
The game-crashing is likely because it was done poorly; a few mistyped characters can really confuse the program. The virus accusations are likely because your anti-virus is paranoid (it's Norton, isn't it? Or Window's warning you about strange downloaded things?)
In any case, try fixing research yourself; go to the Commons folder, open Defines, and change the citizens_to_rp value to 2.2.
I don't do the whole ping-pong fix, so find that yourself. Remember to backup your Defines file first.
