The space bar is the new F8 when it comes to WinPE, Vista and Windows 7.

In Windows operating systems starting at Vista or greater and including WinPE you now have an alternative with more flexible options than just pressing F8 while booting. Next time you restart your system, right after the BIOS POST screen and just before you see the Windows boot screen, try pressing the space bar. If you timed it right or hit the space bar enough you'll now be at the Windows Boot Manager.

You can select which OS you'd like to boot, or you can hit F8 for the normal recovery options you'd expect to see. You can also now hit F10 which will take you to Edit Boot Parameters; this will let you edit the one time boot options. For a more permanent method you'd still want to use msconfig, or in previous Windows versions modify the boot.ini. This would be a quick way of turning on debug mode or verifying it after the fact: [ /NOEXECUTE=OPTIN /DEBUGPORT=1394 /CHANNEL=3321 ].

This can also be used when booting to a Vista or greater setup disc. Right after the system prompts you to "Press any key to boot from CD or DVD". Hit the space bar two or three times. You should get the Windows Boot Manager again and the list should show Windows Setup [EMS Enabled]. You can now hit F8 or F10 in the same way.

If you are troubleshooting a hardware issue you maybe able to use WinPE as the platform to debug.  Note: WinPE 2.x is based off Windows Vista and Windows 2008 and WinPE 3.x is based off Windows 7 and Windows 2008 R2.