physical hd serialnumber

Hi,

How can I get the harddisk physical serial number on all OS's?

I doubt that there is a uniform method to do that on ALL operating systems. Standard C++ knows nothing about harddisks and their serial numbers, so an approach from this side is excluded.

But only on Win98 or higher and using WINAPI or anything else?

Have a look at DeviceIoControl() and the provided examples in MSDN.