Itunes will not install or update

I seem to be having a right royal stuff up with itunes in all aspects, which started with itunes not recognising my tethered devices. After a few hours of research and following online instructions from others having the same problems, I eventually uninstalled and reinstalled.

I d/l 10.6.3 and began installation, but got the "The installation failed" message. I d/l again just to confirm it wasn't corrupt but again, it failed.

To try something different, I d/l 10.3.1 - but same thing. Fail.

Using itunes ( itunes 9 ) from time machine I have attempted to update, but get the "...cannot contact server" message. Also - no recognition of tethered devices.

When I run software update, itunes also does not appear as a required update - even though we know it needs updating as it's version 9.

I'm at a complete loss as to what to do or what's gone wrong.

In an itunes failure spiral.

Running Snow Leopard.