Sure.
First, you wanna download the mod you'd like to install. You should download a bunch of class files, and possibly folders. Not exactly sure anymore, just depends where you got your mod, and what it is.
Second, you want to right click on your minecraft.jar and press, "Open with WinRAR".
Thirdly, you want to DELETE the "META-INF" folder. After you delete that folder, you can copy and paste all of the mod files into the minecraft.jar. (The classes, etc).
Then, start up MC and it should be in. Some mods require different things, so make sure you read if it needs any extra mods, etc.