You need to press F8 before the Windows logo appears.

If your computer has a single operating system installed, press and hold the F8 key as your computer restarts. Click the Start button, click the arrow next to the Shut Down button (or the arrow next to the Lock button), and then click Restart. Remove all floppy disks, CDs, and DVDs from your computer, and then restart your computer. Starts Windows with a minimal set of drivers and services. If you have a Windows installation disc, the system recovery tools are located on that disc. This option is available only if the tools are installed on your computer's hard disk. Shows a list of system recovery tools you can use to repair startup problems, run diagnostics, or restore your system. For more information, go to the Microsoft website for IT professionals. Other options start Windows with advanced features intended for use by system administrators and IT professionals.

If a problem doesn't reappear when you start in safe mode, you can eliminate the default settings and basic device drivers and services as possible causes. Some options, such as safe mode, start Windows in a limited state, where only the bare essentials are started. You can access the menu by turning on your computer and pressing the F8 key before Windows starts. The Advanced Boot Options screen lets you start Windows in advanced troubleshooting modes.

Windows 7 stuck logging off