Close Minecraft

Press Start
Type: %appdata%/.minecraft/bin

Copy minecraft.jar to a safe location.

Right-click the original and select open-archive (or similar) under WinRAR.
Right-click MineFly.zip and open it the same way.

From MineFly.zip
The mods folder in Appdata to the .minecraft folder.

See all of those .class files in the other folder? Transfer those to mincraft.jar

Open Minecraft again. If it doesn't work delete minecraft.jar and copy the backup to the original location.