To check or change the location of the scratch disk, you should be able to go into file->preferences->plug ins & scratch disk.
Here you can specify the hard disk partition to use. I suggest you check that first & if it's using C drive, change it to a different partition with enough free space on it.
If you're still having a problem after that, it sounds like the preferences file may be corrupt. This file is created during startup & contains info contained in preferences section. If you delete this file, it will force photoshop to create a fresh one containing default settings.