itunes for windows freezes/crashes when upgrading to iTunes Plus tracks

After buying the "iTunes Plus" upgrade for several albums that i'd already purchased in their old format, iTunes automatically added them to my download queue in iTunes (running on Windows Vista).

However, iTunes proceeds to lock up after the first file in the queue passes the "processing file" state.

To make things even more frustrating, every time iTunes crashes, it marks 3 downloads as "completed" even though the files aren't on my computer. So now I have to email Apple Support when i fix the crashing problem so i can get those files marked for download again. ARGH!

I tried deleting the old files from my iTunes library first, and that didn't help the problem. Help me, Apple!