Can I use the Windows 7 Home Premium DVD that came with my PC Specialist desktop to install Windows on a new (another) PC/laptop; and then in order to activate Windows, use a Windows Product Key that I will buy from Microsoft?
I believe so, worse case scenario is you can just download the ISO (especially if you need 32/64 bit and you have the other DVD) from Microsoft - I'm not sure if you get it when you purchase an online serial but there are "semi-official sources" (i.e. they point you towards Microsoft's downloads on Microsoft's CDN, but Microsoft doesn't like those links public). My wife actually found it cheaper to buy a Windows serial from Amazon (and got a DVD included: admittedly the case labelling was Germany, but it all worked) than via an electronic serial from Microsoft.