Since you cannot start Photoshop, you cannot change the scratch disk allocation in its preferences. Luckily for you, the engineers have devised a shortcut to fix that: Hold down CTRL+ALT(PC) / CMD+OPTION(Mac) as soon as Photoshop starts to get this menu:
Select another disk with more room but do not forget the clean the files clogging up your hard drive, as described in the other messages.