if so, reset the preferences (crtl-alt-shift during startup. you'll get a box asking if you want to delete the settings, if you don't get the box, you didn't get it fast enough, try again!)
if that gets you in, when you change the scratch disk again, make sure the user you're using has read/write/create/delete (maybe execute?) permissions to the location you choose for the scratch files.
Thanks again
Michael
My permissions are set to Read and Write and I am an administrator.
If you make changes and can't get PShop to open, go to your Library, Preferences, Adobe Photoshop CS4 Settings, and delete "Adobe Photoshop CS4 Prefs.psp"