Kelvin what OS are you using and which version of Photoshop?

If you are using Windows 2000 or newer you will find the Prefs.psp file where Risto stated. The file name will be longer the Prefs.psp will be the last part of the name.

If you have Photoshop 7 the full filename will be Adobe Photoshop 7.0 Prefs.psp.

If you have Photoshop CS or newer it will be Adobo Photoshop CS (or CS2 or CS3) Prefs.psp.


If you are using a Mac then you may need to do a google search to find how to reset the preferences when you cannot start the program.