iTunes is incompatible with McAfee security software.
After a couple of hours on-line with McAfee, letting them control my computer, I was finally able to demonstrate to their 2nd Tier support guy that opening their "security center" did indeed disable this protocol, which was also disable at indefinite intervals by some background process done by the McAfee software, he checked with his superiors and got back to me that ALL McAfee software needed to disable this protocol.
Therefore, if you want to buy iTunes content, I would recommend using some antivirus software other than McAfee.
homebuilt PC, Windows 7, EVGA 780i mobo, 2.83GHz Core2Quad,6GB RAM, GeForce 8600GTS