Sounds like you've developed a Permissions problem or Keychain problem on your Mac.
Safe Boot from the HD, (holding Shift key down at bootup), run Disk Utility in Applications>Utilities, then highlight your drive, click on Repair Permissions.
In Finder do a get Info on...
Folder, do you see any strange rights there?
Open Keychain Access in Utilities, use Keychain First Aid under the Window Menu item, then either check the Password under that item, change it, or delete it and start over.
Then See if this helps...
Mac OS X 10.4 Help, I forgot a password in my Keychain
Resetting your keychain in Mac OS X...
If Keychain First Aid finds an issue that it cannot repair, or if you do not know your keychain password, you may need to reset your keychain.