Do you know of an online back up & sync solution for Windows that actually works?

Solution 1:

DropBox.com ftw. Love it.

PS - Be sure to get the optional Packrat feature turned on. Saves every version of every file FOREVER.

Solution 2:

Maybe have a look at TeamDrive. I am using it to keep around 150GB of files in sync on 4 systems (Win, Mac and Linux) and it works fine, although it is still under heavy development and lacks some polishing and configurability here and there. The best thing, which was an absolute must for me, is that you can use your own server or about any WebDAV space to actually store the data (but I think that requires the commercial version, which is quite affordable).