'My processor fan did stop and overheat the system to a standstill yesturday, before the bad sectors appeared'......I think you may have answered your own question ;-P
If the system overheats, then there will be damage caused to other parts. OTOH, 20kbs is not a lot to worry over, providing it is not near the start of the disk.
I feel that if you took it back to PCW they might be in the right to refuse to change it as there was a related problem. Overheating system = bad news.