Step 1: Cleansing Your Minecraft
If that doesn't work then go to "start", "Accessories" and then click "run".
When the "run" program pops up then type "%appdata%
Next step is to copy your ".minecraft" and paste it where ever you want just in case something wrong happens but i have done this 3 times and nothing bad has happened. Then after you have done that your next step is to delete your ".minecraft" and run the launcher which should update and give you a completely new .minecraft folder.
Step 2: Mod Your Minecraft
To install the mods you need:
-An archive program like winrar (can be downloaded from download.cnet.com)
-Modloader (downloaded from minecraftforum.net)
-The mod you want to install (mods can be downloaded from minecraftforum.net or modsforminecraft.com)
First go back to ".minecraft", double click on it and then double click on "bin" and then open "minecraft.jar" with an archive program.
There is a folder called "META-INF" inside "minecraft.jar", you want to delete "META-INF" otherwise modding your minecraft wont work.
Then you open up "modloader" and copy all the files in it and paste it in "minecraft.jar".
Next you open up the mod you want to be installed and do the same thing.
But some mods are different and require some folders to go into other places but if so it should have a "readme" notepad file, just read it and do exactly as it says.
Last thing to do, is to close "minecraft.jar", open your Minecraft launcher, and enjoy! :D