1. Navigate to your C:\document and settings\[UserName]Application Data\.minecraft\bin in Windows XP or C:\Users\[UserName]AppData\Roaming\.minecraft\bin in Windows 7/Vista
2. Open minecraft.jar using your zip program, I use 7-zip
3. Open up the downloaded texture pack zip
4. Move the files from the texture pack zip over to the minecraft.jar zip. If it asks you if you want to overwrite the files in minecraft.jar click ok
5. Close the zip
6. Play the game
source:http://www.pcgamer.com/2010/09/25/how-to-install-a-minecraft-texture-pack/