For PC: Select the BOOT device in the BIOS. Be sure to use the Proper OS, 32bits or 64bits.
For Raspberry Pi:If the Raspberry Pi hardware is a newer version you may try the .img rather than NOOBS or update the firmware prior to boot.
Otherwise there may be an alteration of the media (SD or USB key) so change media or image and therefore re-download.
The login is linutop and the password is blank (unless you've activated the lock mode).
Yes, in the main menu, select . Change the to Disable Screen Saver.
Use Screen capture application in the Accessories menu
Use the print screen key to take a snapshot of the whole screen, and alt+print screen to take a snapshot of the active window.
You can enable compiz (if the hardware supports it) in the last tab of the appearance setup tool ( ).