[Downgrading Tekkit 1.3 -> 1.2.5]

So, my favourite server is still playing on the 1.2.5 build. But I accidentally upgraded to the newest, 1.3.1 . Any ideas on how to get Back down to 1.2.5?

[When Launcher Opens I get an Error Window]

Anyone?

[When Launcher Opens I get an Error Window]

Launcher Version: Latest - 1.0.1.3

Operating System: Windows XP 32 - Bit

Java Version: 6

Antivirus Program: AVG

Description of Problem:

I click on the launcher, it gets to downloading launcher 3/3 then a small Error window pops up - Error = Error

Error Messages:

Error

Error Log:

None as I haven't got that far for it generating it.