Are you connected to the Internet? If not, you can't. If you are, then you have a Virus in your computer that's attacking it.
I'm having the same problem , my computer wont let me download anything, it will only let me download songs and pictures, but not itunes or messenger?
I had this same virus 2 months earlier.I couldn't do anything with it.I can download music and songs ,but not softwares.I could do nothing but format my computer.