This happens most often after you've started the computer from the CD to reset your account password, or when you've used another admin account on the computer to go into Accounts and change a user password. Once your password has been changed you can login again, but when you open applications like Safari and Mail, it asks you for your keychain password.

The keychain contains your security information - mainly your passwords to your email and other services, as well as a key to unlock the automatic form filling in Safari. If you forget this password, there is no way to recover this information so the simplest thing to do is just throw it away and start fresh.

Click once on your desktop so it says "Finder" to the right of the apple in the upper left corner of the screen. Then select the "Go" menu at the top, and "Home".

This will open a Finder window of your home folder. Locate and double click on "Library". The Library folder will open. Inside that, locate the folder "Keychains" and double click that to open it also.

Inside that folder you will see one or more items whose names end in ".keychain". Drag all of these items into the trash can.

Log out. Log back in again. Try opening Mail if you use Mail for your email. It will ask you for your mailserver password and will have a check box to remember the password. Enter the password, check the box, and continue with it.

Open Safari and browse to google.

At one of those two points it may say "keychain not found, reset to defaults?" Allow that. It will ask you for your password. You are creating your new keychain. Type the same password as what you just used to log in - the password you reset the account to. It may ask you to enter the password several times.

After this, it should not bother you anymore.