Go to Preferences>Plug-Ins & Scratch Disks. Set First to Startup. Set Second to C:. If you have more drives i.e. set Third to D:. Set Fourth to E: or however your system is set up. Press OK. Press Control+Q to exit Photoshop. Exiting in this fashion will save the settings. Open Photoshop again and you should be fine. Also, occasionally if you have a problem, double check this area to make sure your program wasn't reset some reason. Your problem should go away.
Also, on occasion, people have trouble with their system resetting due to newer versions of software especially in Windows 2000. If this happens, disable your Anti-virus program while working in these programs. Just make sure the anti-virus is on when on in email or the internet.