Many players have updated their client a bit to fast (judging from the massive amount of mails I have got the past few days). This is a way to revert / downgrade your minecraft client back to version 1.2.5 from any newer versions 1.3.x

First locate your .minecraft directory – Click Start and enter %appdata%
This will open your AppData directory – locate the .minecraft directory, enter it

If you have Version History on your windows machine, you can simply right-click the bin folder and select Revert back to earlier version and select a version on the list from a time when you had 1.2.5 installed

Alternatively you can delete the content of your bin folder and replace it with the content from the ZIP (zipped bin for 1.2.5)

Start you game again and this time say Not Now when asked to update to 1.3.x

Replace all the files inside