Problem after downloading from iTunes

Hi, your assistance please! I have the computer program Windows XP 600GB HD, 4GB rm and it’s been working well before.
The problem arose unexpectedly after I made some downloads.
I tried to download at iTunes but the download stopped halfway. Afterwards, I got the following prompt messages: “The older version of bonjour could not be removed” and “Apple Software update could not be removed’.
From then on, I was unable to install and download music files from iTunes and also, error messages with filename “MSI” would appear. By the way, what is MSI? What could have caused the errors?
How could I troubleshoot this?
Thank you.
