Hey everybody,
I also was experiencing the unknown error -42018 when attempting to download purchased music. Additionally, I was stuck in an endless authorization loop when attempting to play previously purchaed music. All this started when I installed the latest iTunes software 7.0.2.
I run windows XP Pro, and unlike the majority of users I actually set it up correctly with a dedicated administrator account, and regular user accounts instead of just running as admin all the time. For me,
that was the cause of the problem.
The new version of iTunes software required FULL CONTROL privilages on the iTunes and iPod install directories for all user accounts that were using the program. (Previous versions worked fine, so this is a new thing.) Once I provided those permissions, iTunes worked correctly. Follow this link:
http://support.microsoft.com/kb/304040 for information on how to provide FULL CONTROL to every user account.
I hope you all find this helpful in solving your problem.
Good Luck,
Bill