I have had a couple instances where I downloaded a program or update off the internet but it wasn't from an incredibly stable source, and with the download came some weird secret program--malware.
This would cause every single internet window to have popups, no matter which site....everything, all the time.
The first time I had to do my homework and after a day I found it to be this secretly added software(malware), and both times all I did was go to my control panel, look at my list of programs with regard to installation date, and WHAM!
Both times there was some oddball program I totally didn't recognize/authorize added within the last day. Once it was completely uninstalled--and I had to go through some files manually(instructed by a guide on internet)--the problem disappeared.
But is this same scenario even possible for MAC's? Im not sure how malware type stuff works for apple.