Removed messages, how to get it back

Hi all,

since a couple of weeks I got this strange error in messages: whenever I tried to login, messages would log me out automatically without any visible error or feedback. I then decided to remove the application using an uninstaller, little did I know that there isn't a standalone installation file of messages anymore.

So.. how do I install messages again? I'm currently downloading Mountain Lion, but I'm not sure if this will erase my whole system or is also capable of doing a restore for missing elements?

Thanks.