First, I'd power down the PC by using the button on the front of the PC. On restart, you'll be asked how you want to start Windows. Choose last known good configuration. If that doesn't do the trick, try the below.
Try hitting the F8 key on startup to access the options such as Safe Mode, Safe Mode with networking, etc.
Choose the top one, repair my PC, choose the Startup repair option first. If this doesn't work, you may have to use System Restore.
By the way, I've done this numerous times on my Windows 7 machine.