My Documents redirection path

I am setting up a SBS for a friend who bought a pre-installed Dell box and he wants to have My Documents redirection enabled.

In our office I set the server up from scratch so specified the UserData partition at install so when I enabled My Documents redirection it automatically saved the data on the SBS in E:\.

However on the Dell machine I am setting up it has 2 partitions, C:\ with the OS on and D:\ that was empty but I want to use for all other data.  When I enable My Documents redirection on this machine it starts putting all the files into the C:\ partition.  Where are the settings to force it to place the files in to D:\ otherwise my OS partition will fill up!  Is there something I change in the registry or group ploicy or is the only way to do this is to create a share on D: drive and select "Redirect all My Documents folders to a network folder"

Thanks