Go to C:\documents and settings\currently signed in user\application data\.minecraft\bin, backup the file named "minecraft", after you're done with the backup, replace the original minecraft.jar with the 1.8 pre-release minecraft.jar, start the game, and there you go! If you want to play the previous version again, do what I said, except that this time you replace the 1.8 pre-release with the original one.
