How do I approach thermal pad removal, clean up?

[bulletsneverlie]

Upon inspection, I see the fan has a lot of crusty silver stuff on the heat sink surface. I pulled on the silver stuff, and it came off in a square, which appears to be some type of thin metal covered in a crusty substance. There is also a thin plastic ring I removed that held the silver square on. Is that ok? There's a little bit of silver stuff still stuck to the copper matting surface of the heat sink.

Ok, before I put on thermal grease, do I need to clean off the rest of the silver on the heat sink? How do I do that? I tried 91 percent alcohol, but it doesn't work. Can I use a 1500 grit sand paper? Also, do I need another silver pad, or just the thermal grease?

I have an IBM r40, p4 processor.