If you can not access the tape drive from your Linux box, you can FTP them to the Windoze box and back them up in Windows, or if you can access the Linux drive from Windoze, you should be able to map a drive and have a backup script to back up only the file(s) you want.
Most good backup software has clients for Linux as well as Windows, etc. You can download a trial version of Veritas Backup Exec 9
that contains a Linux client.
There are plenty of serial #s floating around as well for it.