Why won't my laptop allow me to download the new version of iTunes?

I already have iTunes, I think its 8.2, or 8.0 or something. I tried downloading iTunes 9 (the new one) and its saying that it is an “invalid application” even though I turned off my pop-up blocker! I need to download this version ASAP because iTunes is saying I need the new version before I can buy anything off the program (music/ TV show episodes). Does anyone know how I can make it so it will allow me to download iTunes???