Reinstalled Windows via original cd of SP2 Professional without formatting the harddisk and overwriting the current version. It felt like computer reborn. But after the day of reinstallation the problem showed itself again...
I finally got it It was the DUST( or maybe just something similiar )...
I opened up the cover under the laptop and cleaned it via vacuum cleaner...
The problem %100 got away. It was the HEAT which slowed down the computer after some time from the beginning of rendering...
