Very annoying update problem! Please help!

I had to do a clean reinstall of OSX on my MBP so when I go to update to 10.4.10 or nine or whatever I get this message:

The digital signature for this package is incorrect. The package may have been tampered with or corrupted since being signed by “Apple”.

What does this mean and how do i fix it?
iTunes requires 10.4.7 to run, I am so screwed.