The HP Branded ones seemed a possible route
The branded ones are generally BIOS locked I believe.
I'm still concerned what's going to happen when I upgrade my HD
I think you'll be safe with just a HD upgrade, as it's probably seen as a minor system component (similar to RAM) that doesn't really materially alter the system in the same way that changing a motherboard or processor would.
But don't quote me on that just yet, will do some digging around and see what I can find in terms of more official documentation on the matter.
Update:
Document from the Microsoft website for XP, but I suspect things are largely the same, just more rigorous and complex for Vista/7 ..... however the document should give you a good feel for how things work....
http://download.microsoft.com/downlo...etbulletin.doc
Also may be of interest
Quick start: Activation FAQ