With AutoComplete 'on', your IE prompts you whether to save the logon name/password or not. If you select 'No' your decision is recorded in the registry and you wont be prompted again ! But what if you change your mind ? Here is how you can reset.

Run Regedit > HKCU\Software\Microsoft\Internet Explorer\IntelliForms\Storage2 > See the left pane > select the Storage2 key > Click File > Export > Save the key to a safe location as SavedPassword.reg Now in the right pane, select all values & click Delete.

Now in IE, go to the site whose password you want to save. Fill in your details and then click to submit the data. Now when IE asks if it should remember your password, click Yes.

Come back to the Registry Editor. Click File > Import > Select the file you saved as SavedPassword.reg > Click Open to merge the saved data into the registry.