Time Machine Backup to a Windows computer
There was a lifehacker article on doing exactly this by creating a sparse HFS+ bundle on the Windows machines.  You could use a Linux machine or Amazon S3 using s3fs + encfs too. 

Time Machine requires an HFS+ filesystems backup disks like this sparse bundle because Time Machine uses Apple's unusual directory hard links.

Linux could theoretically read the HTF+ sparse bundle but Windows sounds doubtful.