The serial number on your hard drive is indeed tied to the oem activation sequence for that machine. ( More probably, that type of machine )
It doesn't go online to activate - that is the way oem's do so many machines - they can't enter each product key one by one onto millions of machines.
It is possible that same number, activation cert and bios slic has been used for cracking - it happens.
If you want to clean reinstall without the bloat - you should use the completely different product key that is somewhere stickered on your machine - often under the battery.