I took the following little guide from here:
http://www.minecraftmine.org/how-to-dow ... minecraft/
• Download a clean Minecraft 1.4.2 Jar file:
http://assets.minecraft.net/1_4_2/minecraft.jar • Type %appdata% into the start menu
• Open .minecraft folder
• Open the bin folder
• rename the current Minecraft.jar file to Minecraft-bak.jar
• Paste in the clean Minecraft 1.4.2 jar file
• Done
I have not tested this, as I have a Mac  And also with 1.4.2 still installed, as I never upgraded. But by the comments, the site seems reputable. Follow the above and you should be able to downgrade to 1.4.2 pretty easily, though do so at your own risk.
The link for the 1.4.2 jar file I took from a post elsewhere by Jak2131.