If I..

[nmdride]

Install windows 7 on a hard drive and decide to upgrade to a SSD in the future will I need to buy Windows 7 again to install on the SSD? Or can i still use the same CD key

 

[mdd1963]

you can use the same one for when upgrading/replacing internal components such as drives...; very commonplace.

 

[joelmartinez]

Microsoft only really gets pissed when you switch the mobo

 

[Zenthar]

And OEM Windows license is tied to the motherboard, not the hard drive it's installed on; no such limitation for Retail. If your MB was to fry and you had to change it, you could probably call them, ask them to reactivate and they would probably don't make much out of it.

Moreover, when switching to an SSD, you don't necessarily have to reinstall everything. If you have enough space on the SSD, you can just clone one to another (that is what I did when I switched).