Windows xp installation:
1. Download a mod and if it adds stuff to the game like mobs and items or blocks then download modloader!
2. Go to start menu and at the right side of the start menu you should see a program called run and if you don't then press R and type %appdata% and Press enter.
3. Go to .minecraft and goto the folder "bin" and open minecraft.jar using any archiver program
4.Put all the class files into minecraft.jar

Windows 7 installation:
1. Download a mod and if it adds stuff to the game like mobs and items or blocks then download modloader!
2. Go to start menu and in the search box type %appdata% and click on Roaming.
3. Click on .minecraft and click on bin and open minecraft.jar using any archiver program.
4. put all the class files into minecraft.jar