FlutteryChicken

Glad you got it working, I did'nt realise you could download the older versions from minecraft.net otherwise I would have told you to do that.

If you have the 1.2.5 try putting it in the '\AppData\Roaming\.techniclauncher\cache' folder

That's the problem I had, it's just because the minecraft.yml file has not been updated by the technic team yet.

I fixed it by having a copy of 'minecraft_1.2.5.jar' in the 'AppData\Roaming\.techniclauncher\cache' directory.

As far as I can tell it is because the new vanilla version has been updated to 1.4.2. So hopefully it wont take them long to put the checksum of that version in the launcher. If you have not updated your vanilla minecraft it might be possible to use it. But it shouldnt take long to get this sorted.