Ok, I've done some more tests. I can't find a /etc/default/urbackupclient on my Linux installation (I've installed using the .sh file provided), so I don't know how to set this option. I've found no man page, too. Tests done using Arch Linux.
About Windows, I can see that, for example, c:\Documents and Settings is backed up, but no hashes are generated, so I can't restore anything.
If I create a file on c:\, it gets backed up and I start the restore process from the web interface, the pop up appears and everything seems to be done, but at last the recovered files and/or directories don't appear at all, so it seems that the restore isn't performed at all.