Non-XP systems will be using SAMBA as the connection technique for file/print sharing
(ie: ports 139,445 and lmhost v1 authentication)

XP (and Vista) have two authentication techniques
LMHASH and NTHASH

SAMBA access will require LMHOST V1 authentication
(it is possible to use V2 *if* all non-windows systems support V2)

Windows XP/Pro or Vista/Ultimate users who have set the Global Policy

Network security: Do not store LAN Manager hash value on next password change

located at

Computer Configuration, Windows Settings, Security Settings, Local Policies, Security Options

will not be able to use filesharing to non-windows systems.

The alternative tecnhnique is a regedit

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa
click Add Key, type NoLMHash, and then press ENTER.

The GPO is preferrable, but XP/Home users must use this regedit to inhibit LMHASH

While it is true that setting this policy enhances the Windows security, it has a negative effect
on Sharing to non-Windows systems.

see the Microsoft KB article
http://support.microsoft.com/kb/299656

btw: this policy also stops LMHASH password cracking
so you choose Security and no sharing or Sharing and compromised security

On a side note:

I confirm that LMHASH cracking on this security entry (and also on encrypted drives) can cause more ill effect. And therefore may not be the wisest choice of offered support in this. (sadly )
Mind you, if you had mentioned that earlier, I would have agreed from the start.

BTW I'm still confused why Vista (sometimes) has issues with shared drives/printers on Xp But that's not this thread's relevance.

I'm not 100% sure of the 'combination to this lock' myself -- there are sever new techniques
and the settings are not readily obvious. Without a copy of Vista, I can't play with the
network or the GPO/Security options other than to read MS KB articles.