Question: I use windows and now ubuntu and want to use ubuntu one to backup and sync files between them both so no matter what OS I use, I have the latest synced files. Is that possible?

Answer: 
Yes, there is [Ubuntu One client for Windows](https://one.ubuntu.com/downloads/windows/).
I believe it should be possible.

Just install [Ubuntu One](https://one.ubuntu.com/downloads/) on both Windows and Ubuntu.

Say you have a folder `Documents` in Ubuntu and you want it to be synced between Windows and Ubuntu. Then just select Sync Locally for that folder in both the Ubuntu and Windows - Ubutnu One Client.

![enter image description here](https://i.stack.imgur.com/TYfFB.png)