In Windows 7 especially, but also seen in Windows XP and Vista, it is quite annoying when the credentials of a share have changed and Windows will still be trying to connect using a (possibly) no longer valid username/password that you have previously saved.

To get the username/password dialog box to show up agian, the first thing you can try is to delete the share connection via NET USE as shown below:

C:>net use
Status       Local     Remote                    Network
-------------------------------------------------------------------------------
OK                     \\servername\IPC$         Microsoft Windows Network
The command completed successfully.


C:>net use /delete \\servername\IPC$
\\servername\IPC$ was deleted successfully.

First line shows the shares you are currently connected to. The second line disconnects.

If the above fails, you may need to reset the saved credentials. In Windows 7 (and Vista I believe) the tool needed to do this is called netplwiz which you can just invoke from the Start | Run button.

In XP, netplwiz.exe does not seem to be available, so you will need to invoke a similar utility via rundll32.exe as shown below (should also be invokable via Start | Run:

rundll32.exe keymgr.dll, KRShowKeyMgr

Now when you connect to a share, it should ask you for the username/password again. Sometimes you will need to logon/logoff before this fully works.