Try adding the domain= option like so:
CIFS and SMBFS perform essentially the same function. SMBFS is no longer maintained and has depreciated. CIFS is being maintained and is replacing SMBFS. SMBFS has not been a part of Ubuntu since Gutsy.
//SERVER/Documents /media/Server/Documents cifs credentials=/root/.smb_pwd,domain=WORKGROUP,rw,nounix,iocharset=utf8,file_mode=0755,dir_mode=0755 0 0
SMBFS is also a meta package, which means that when you "sudo aptitude install smbfs", you're installing several SAMBA related packages including CIFS and winbind. CIFS has been a part of the SMBFS metapackage since Breezy. Interestingly, the actual SMBFS package has NOT been a part of the SMBFS meta package since Gutsy.