NadaSurf,

Its an easy fix bro...In Windows Control Panel, select USER ACCOUNTS, click on CHANGE AN ACCOUNT, click on the account you use to log into your computer, and then on the left click on the little link that says MANAGE MY NETWORK PASSWORDS. You should now be presented with a list of accounts, some of which are the accounts listed in the .NET login. And a Remove button...click remove and update...you should be all set.

I assume that you are using Microsoft Internet Explorer.

1) Select the "Tools" menu
2) Select "Internet Options"
3) Select the "Contents" tab
4) Click the "Autocomplete" button
5) Uncheck the "Web Addresses" check box
6) Click "OK" on all boxes until every dialog is closed

This will work if your wife is not computer savvy, if she is, you will have to click the "Clear Forms" button below the check boxes.  Be advised that this will clear all forms, not just the one you wanted.

I personally have never used MSN Messenger (I prefer Trillian from:
http://www.ceruleanstudios.com/  which has a password option for each account that has each platforms login info.....so no password to the account and no one can see the logins)
So as to where the information is located I can only guess,
you might try: C Drive > Documents and settings > your user name >
Application data> and then see fi there is an ".ini" file for  MSN messenger there, open the ".ini" with notepad DO NOT OPEN IT IN WORD OR THE LIKE!!!!

Now, if you want to turn off all auto complete in Messanger you can download this registry edit and run it (just double click the file):
http://messenger.jonathankay.com/downloads/disablenetcreds.reg

or go to:
Start
Run
Enter: gpedit.msc and click oj
Computer Configuration
Windows Settings
Security Settings
Local Policies
Security Options
"Network access: Do not allow storage of credentials or .NET Passports for
network authentication" double click and set to "Enabled"

Hope this helps.

You may also want to setup individual computer login accounts that are password protected. With doing that each user has their own respective areas profile, caching, and file storage areas.