Windows 7 Pro "Upgrade" on blank HDD

I'm looking for a little input.  I'm getting ready to build a new computer.  My OS is Windows 7 Pro 64 bit, but I purchased this as an upgrade from back in the day I had Windows XP.  I have both serial numbers, but was hoping not to pollute a new clean HDD with old OS files.  Some Googling suggests that I can make a change in the registry to make the system believe there is an old OS and therefore continue the activation without problems.

I've seen this discussed in articles/interviews with Microsoft and I do legtimiately own XP, so it seems that this is both possible and legal.  Can anyone given me any thoughts, insight or problems I might encounter doing this?

I sure wish they'd let me install a clean Windows 7 and then just provide both my XP and Windows 7 key to prove the upgrade is appropriate for activation.