There's been mixed opinion here about the use of Windex. I'd recommend
against it - the ammonia.
Generally, a very soft non-linting cloth (no lens tissue!) and just plain water should be able to remove the vast majority of the gunk that's removable.
If you
absolutely need a more active solvent, if I remember properly, NIST recommends as a sort of last line of defense to be an alcohol based cleaner.
And, of course, wipe
across the data track, not along it.
In a pinch, I've often used (a) a relatively clean cotton or cotton/poly shirt, and, as needed, (b) a bit of Universal Museum Solvent.
Leo Kerr
P.S. What do you mean, you don't know where to get UMS?! From a relatively clean mouth, spit on your hand, lick your finger, whatever, and use it! 
