Password changes for Active Directory (and Novell and Midd-Unix, aka community) can be made at http://go.middlebury.edu/activate
On college-owned computers, it may be advisable to logout immediately after a password reset, then logging back in with the new password.
NOTE: passwords can be stored by numerous means on a computer. Use of the old password multiple times can lock an account. It is recommended that passwords NOT be stored. If they are, you may need to clear them by whatever means necessary.
Mac OS X stores passwords in the Keychain. From the Finder, select Utilities from the Go, then open Keychain Access. Make sure you can see the list of keychains (View menu), and select the "login" keychain. You may be interested in entries referring to Exchange, mail.middlebury.edu, midd_secure, and the like. It may be advisable to simply delete such entries.
Windows may also have passwords stored for midd_secure or other connections.