Log in as an admin, and go to Control Panel.
Click on Users and Accounts, then click on Change the way users log on or off.
Then click the box next to both Use the Welcome Screen and Use Fast User switching, and click Apply Options.
That should fix it.
What's a mystery to me is how it changed of its own accord, I really don't know. Spyware?
Master of common sense. If you don't like it, stop reading.