Webmin doesn't seem to be able to browse for cifs shares on a WinXP box in disk and network filesystems using the cifs share mount dialog boxes in disk and network filesystems. The problem seem to be that Webmin uses anonymous browsing to discover the shares but XP won't return any results unless one uses a valid uname/passwd combo for an account on the XP box. The same result can be observed using command line smbclient.
There is no problem for Linux boxes or for Win7 boxes, they return a share list fine. It's not a problem either accessing the cifs share and mounting it in disk and network filesytems, I can enter the share name manually and mount the cifs share properly, only it can't browse for the share names. Webmin browses for shares with smb_share.cgi. The relevant line of code is;

&execute_command("$config{'smbclient_path'} -d 0 -L $in{'server'} -N", undef, \$out, \$out);

It's the -N switch that sets no password needed i.e. anonymous browse. I made sure there is a guest account enabled on the XP box with no password and added it to the shares permissions as read-only. This did not work.