samba - Mar 2006 - Two local master browsers in the same subnet??

Hello I have the following problem. I have a Windows network, when Samba
starts it says in his logs that is local master browser for his workgroup.
However when I do nmblookup -MT <Name of the workgroup> it returns two ip
addresses, one of them is the address of the Samba Server, the other is the
address of a Windows machine (could be Windows 2000 or XP). The thing is
that the browse list that is seen in the network neighborhood is the one
mantained by the Windows machine, so for all practical purposes the local
master browser is the Windows machine. If I reboot the Windows machine the
Samba server becomes the local master browser and everything works ok (doing
an nmblookup returns only the ip address of the Samba server).
Any ideas why is this happening and how to fix it??
Thanks in advance.

Pd: the following are the relevant options of my smb.conf (there isn't any
other Samba machines)

local master = yes
os level = 65
preferred master = yes
browse list = yes
domain logons = yes
domain master = yes