There have been reports regarding problems with iTunes and Windows Vista. Now that Windows Vista is released, Apple is addressing such issue.
iTunes Repair Tool for Vista 1.0
About iTunes Repair Tool for Vista 1.0
The iTunes Repair Tool for Vista 1.0 will repair permissions for important files required by iTunes to play your iTunes Store purchases.
About iTunes and Windows Vista: http://www.info.apple.com/kbnum/n305042
Apple offers some informations regarding iTunes installations on computers with Vista upgrade and new computers with Vista pre-installed.
Apple also mentions that new updated version on iTunes is on the way.