Steve that is excellent advice.

Thorsten, you should do all those steps with EA held low, preventing internal code from running.

Also read Doc0401.pdf Page 20 and Doc1486.pdf.

See DOC0898.pdf for Atmel's version of a serial - programmer.

What is the programmer you are using ?

Is this a development board ?

If the chip is socketed, try putting a known blank one in. Digikey sells them. They are around $6.00 each. You should have some spares anyway. If it is soldered in, then hold off on removing it as a last resort.