iDisk Utility for Windows XP?

Yep, it's available from this .Mac web page. According to Apple,

"New iDisk Utility for Windows XP makes it easy for friends and co-workers to upload or download files from your iDisk Public folder. Plus, you can access your iDisk from a machine running Windows XP. Send friends and co-workers this URL where they can download their free copy of iDisk Utility for Windoes XP: <http://www.mac.com/1/idiskutility_download.html>"