Trying to download itunes update on my computer windows Xp... Tried 3 times and keep getting an error: invalid signature/will not be installed. HELP

iTunes is downloaded, been using it for years. I went to download an update and I keep getting an error.Error message is: invalid signature/will not install.

I did try to update manually as instructed however that also came with the same error.

If someone can advise it would be greatly appreciated.

So what is an invalid signature and how to fix it???