So I decided to install Mozilla, and I kept getting the same invalid signature problem to intall it! When I found out your website (which had a similar topic but did not seem closed), I decided to start the step-by-step malware cleanup and I couldn`t even download the temp files cleaner, as I was also getting the same message on invalid signature!!
I have Avira AntiVir and they did not detect any problems, although I am not able to update it (error message without any specification)
So, what turned out to be a itunes problem seems to be a problem with my PC, but I cannot solve it....if anyone can, please let me know!!! thanks!!