Try toothpaste.
Yep, not a typo this time, I said toothpaste!

Apparently it is slightly abrasive, so helps clean off surface scratches. I tried it on a couple of my sons CD's that he scratched, and it worked well enough to let me copy the contents of the CD off.
Just make sure you rinse it off really well, and polish the CD dry with a clean soft cloth before putting it back in the player. I'm failrly sure it can't hurt the CD, so why not? It's free!

If that doesn't work, buy one of the commercial cleaning kits and try that.

Cheers
Mick