I have used Windex on a paper towel to clean some DVDs.
Mostly only if they are very dirty.
I have used the radial motion, and nothing has happened to make the disc unreadable.
In fact several of those store bought cleaning kits use a circular motion to clean.
The "How to Clean Things" site I linked to above suggests Windex (well, window cleaner anyhow), although it states that paper towels don't pick up dust and debris and therefore the dust and debris can scratch the disc as you wipe. Apparently microfiber and 100% cotton pick up dust and debris well. As far as the store bought disc repair devices go, I've heard bad things. I've heard good things however, about taking them to resale shops etc. that have a professional disc cleaning unit for a small fee. I wonder if Mega Media Exchange provides that service. I know they have the unit for "in house" cleaning. On a side note, I read something really quite weird at the "How to Clean Things" site that I've heard of before in the past. Let it be noted that I am NOT suggesting anybody try this. "Don't try this at home kids." It suggests using abrasive toothpaste as a means of buffing out scratched discs. It seems to be as a last resort and also suggests that you take it to a professional instead, if you're apprehensive.