Installing Optifine Minecraft On Windows

Locate your minecraft.jar file. On Windows, its in %APPDATA%/.minecraft/bin 

Create a backup of minecraft.jar 

Open minecraft.jar in an archive editor (WinRar/7-Zip/etc) 

Delete the META-INF folder. 

Copy the .class files from the downloaded zip file into the jar file, replacing previous files. 

Run Minecraft and test! 

That's it, there really isn't much to installing the mod.