Copied from Jetscat:

Download a clean Minecraft 1.2.5 Jar file
Type %appdata% into the start menu
Open .minecraft folder
Open the bin folder
Delete the current Minecraft.jar file
Paste in the Minecraft 1.2.5 jar file
Done


You will need to find a clean 1.2.5 jar file on the net, it shouldn't bee too hard.

There was also a link going around to a reset website/forum post where you could basically un-patch MC but I don't have it handy.

RagMan