Hello,
to solve your problem please follow these steps. You require a programme like WinRAR to do this!
1. Open up your explorer and go to %appdata% in the navigation bar
2. Delete the folder .minecraft
3. Start Minecraft and launch the game once
4. Now download modloader here: http://www.minecraftdl.com/modloader-download/
5. Open up your minecraf.jar by going to the .minecraft map again and then go to bin
6. Delete the META-INF folder from your Minecraft.jar
7. Open up your downloaded modloader and drag the contents from the folder into the Minecraft.jar
Thats it! Your done, modloader is now succesfully installed.