Windows 7 and subsequent hardware upgrade

[foreright]

Beacause of the end of XP support I intend to upgrade to W7. I chose that OS as I have an mAudio sound card which have drivers for W7 but not W8.
I originally intended to upgrade my PC at the same time but for varying reasons intend to wait a while on the hardware upgrade.
My concern is what happens at a later date when I do said upgrade and re-install W7. Am I allowed to use the same licence as the PC will appear as a completely different spec to Microsoft.
Can someone please tell me the legalities and offer advice. |Thanks.

 

[foreright]

I did that on an XP laptop but wouldn't want to on this PC as it is used by the family and I'd like a MS mail reader live Live Mail so I can transfer contacts and messages easily from Outlook Express.
I hadn't decided on retail or OEM and would be happy to pay the extra if it meant I could use it again after the ugrade which unfortunately would be MB and CPU +

 

[foreright]

Get Win7 Retail.

That is the answer except I can only find it on a rival site at £!54. Was it always that price - I can buy 2 OEM's for that!