After Activating windows, will the old serial be available for use?

I was installing a new graphics card a while ago for my PC and was pretty sure that I did a good job. Anyways Something wasn't right, because my computer refused to load windows after i started the PC up. As if windows wasn't even installed. it was asking for a CD/DVD to boot from. I checked the BIOS settings and made sure that it would boot from the hard disk first. I was looking around for someone with similar issues and it was suggested that it simply might just be a loose cable. I double checked everything and all was well. Except the fact that I couldn't find my original windows CD, so I purchased another. After a couple of weeks I happened to stumble upon my old original windows 7 CD, but I already used the new serial number from the new one. Now my question is, can I still use the old serial that is no longer used on another computer? I mean, will the old serial be still available since it's no longer used? or is it gone?

Thank you for the swift simple reply. Much Appreciated.