Windows 7 – Fixing The Mapped Drive Credentials Problem

In the Group Policies Editor, open each of the following items in turn:
Computer Configuration
Windows Settings
Security Settings
Local Policies
Security Options
Now, right-click on the “Network security: LAN Manager authentication level” policy item, and then, from the context (pop-up) menu, select “Properties”.
Now select the “Local Security Settings” tab, and then, in the dropdown box, locate and select “Send LM & NTLM – user NTLMv2 session security if negotiated”.

from [url]http://www.gadgetgrill.com.au/2009/11/19/windows-7-fixing-the-mapped-drive-credentials-problem/[/url]