I can access my NAS share using Windows XP but I cannot access a samba share from Vista. It asks for user name & password but then still it won't allow access. I have another samba server on the network which I am able to access and it doesn't ask for user/password. Any I am using ADS.
Try to run secpol.msc
Go to: Local Policies > Security Options
Find "Network Security: LAN Manager authentication level"
Change Setting from "Send NTLMv2 response only"
"Send LM & NTLM - use NTLMv2 session security if negotiated"
Vista defaults to only send the more secure NTLMv2 protocol, which some NAS devices / Samba do not support.