This weekend, my laptop came down to an agonising slow speed. It was getting heated too much. The voice of the fan had also slowed a lot. I decided it was time to call Dell again. So I called them Monday and explained the problem with my Dell Inspiron 1521. The guy agreed to replace its motherboard. The next morning, a technician came with a motherboard, fan and heat sink unit. He laid out a rounded sheet and plugged its wire to the power output. He wrapped another of its wires around his wrist. I asked if it was a protection against electric shock, but he said that it would protect the notebook against any static charge. he opened up the complete notebook and replaced the three parts. I asked him many questions. He mentioned that the typical life of a notebook is 6-7 years. He also showed me a ball of thready dust (about the size of a big cherry) that was stuck in the fan. He told me that the notebook has a protection mechanism such that it reduces the processor and graphics processor to reduce the heat, if fan or heat sink is not working. If that doesn't work, it shuts down the system.
After that, i turned on the system and I was able to watch youtube videos in fullscreen again. My warranty would expire this year. I guess I'll renew it to get my notebook parts replaced often :)