You can do something comparable to this with just about any OS if you have physical access to the box (as booting into single user mode requires on OS X) and th

You can do something comparable to this with just about any OS if you have physical access to the box (as booting into single user mode requires on OS X) and the BIOS has no password protection. E.g., Boot from a flash drive that can mount NTFS. Now you have full access to your Windows file system. Encryption is the only solution here (a BIOS password doesn't stop someone with physical access from removing your disk), and OS X supports that as well.