Not sure, I have Verizon and no problems, others have Verizon and problems.
ZA tells you when it blocks things and your other programs wouldn't be blocking iTunes without telling you as well.
You've done Ctrl-Alt-Del to see what's running and checked Event Viewer?
This is one of those occasions when I would back up the music library, repair XP and reinstall updates/drivers with internet testing.
If everythig was all right I would install Quicktime stand alone first and, if it worked, I would then install the latest iTunes.
It's too late for you to use System Restore so that looks like your best option barring a reformat.
You shouldn't have to go through this but some times it's the only thing that works.
Asus M4A77D, 64 X2 6000+, 4 GB Corsair DDR2 800 ram, Radeon 5770.