Why can't I download iTunes?

Hey guys, ok I already had iTunes and you know how it always says to update to the "new iTunes 10.3" or whatever. So I did and it was installing it was almost done when it said something like "an error has occurred". So ok I did the update again did the same thing I have tried downloading it about 4 times. And now since it exited iTunes i can't get back on it because if i try and re-open it it will say "iTunes cannot run because some of its required files are missing. Please reinstall iTunes. " Now im thinking im going to lose allllll of my music but i have it saved on a file but not my new music that i got. But o well. So i go and reinstall itunes. I download it and it starts installing, everything is going fine it says gathering computer space requirements and then BAM again another little note that says " A network error has occurred while attempting to read the file ...." i have also tried to re-download it several times and nothing. Please guys what do i do. I had so much music on itunes and i want to transfer it to my new Ipod touch, please help what do i do.