understated

I downloaded Abrosia a week or so ago, it was all working fine but then my computer suddenly came up with some spyware crap claiming to be anti-virus, when I clicked it off it immediately just popped back up. Had to do a roll back to the day before I installed this program for my computer to function properly again. I'm just a bit confused - obviously loads of people have downloaded and are using this program without a problem. This was the only program I installed that day. Any suggestions as to why this happened? I liked the program and would love to use it!