Enable Organization Admins to Login as Any User

Go to Administer | Security Controls | Login Access Policies | Select Administrators Can Log in as Any User

With this feature enabled, System Administrators can log in as any user in their organization without asking internal end-users to grant login access.

Note: This feature is available in Enterprise, Performance, Unlimited and Developer Editions. If this feature is not found in your Org, contact Salesforce.com support to enable.