If you don't have the original version of Windows installed that is associated with the key on the computer, then you should have the key of the version that was installed written down or stored somewhere. If you never got it, you should have saved it somewhere before reinstalling that version as there's no way to retrieve after a full format and reinstall. At this point, you have the following options:
1) Purchase a key for the current version installed
2) Continue using it through the end of activation with the limitations
3) Install a different, free OS
So, it's just up to you what you want to do at this point. Lessons learned. You need to make sure you save the key prior to formatting next time. You should also have a copy of the current OS somewhere as a backup, with the associated key written on the CD or stored with it. You shouldn't purchase a computer through a reseller without a legitimate copy of an OS with a valid key. You should also not use other people's cd's to install a new OS that you know you don't have a valid key for and expect to find a way around it. Those days are over. Microsoft caught on to multiple OS installs with the same or "volume" keys. I know it's expensive, but that's just the way it is these days. Do it once and do it right and then you don't have to worry about it for a long time.