The /U is the switch that tells it to use the (default) unattend file. The
/U: is required if you want to give it the path to an unattend file.

To see the other options you can use, open a command prompt, go to the
directory where setupnw.exe is and type setupnw.exe /? and you should get a
dialog box with the other options.

There may be an option or two in your unattend.txt file you need to change
so the install runs without any user interaction at all, but you might
already have those settings right.