This fixed the problem for me, that I couldn't log on to the Leopard 10.5.2 SMB share from Vista (SP1). I grabbed the solution from http://forums.macrumors.com/showthread.php?t=377771 and I am archiving it here for me to easily remember.
Use regedit and change \HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa\LmCompatibilityLevel from 3 to 1.