You could mount your iDisk as a mapped drive on your Windows PC, and purchase something along the lines of Acronis True Image.
However, what's the point? There are several free and super-cheap iDisk solutions on Windows that come with great backup software.
I would recommend Microsoft's Live Mesh sync service. You can designate different folders for it to sync "to the cloud." And access those files from both your Mac (via their new Live Mesh client) or mesh.microsoft.com. I believe you get around 5GB quota for free.
Also, JungleDisk is great (and cheap) backup software/service solution. It backs up automatically to Amazon's S3 service (15 cents per gigabyte per month).