Itunes wont download?

I recently bought an ipod touch and I had to update my Itunes and it required me to delete my old one, so I did. Now for some reason I cannot download it (I've had problems with downloading the new Windows Live Messenger too). I dont have a firewall blocking it or any of those stupid standard reasons that it assumes may stop you from downloading it. The most recent time I tried to download it, it told me i need to be administrator of the computer, which I am?? Please help this is very frustrating!! :(