I recently bought a cheap second hand laptop on ebay. It came with Windows 10 Pro installed, but no installation media. If possible I would like to downgrade it to Windows 7 because the manufacturer fully supports this version and for example, some hot keys don't work in Windows 10.
I have asked the seller if he upgraded it to Windows 10 from a version of Windows 7/8 that he had bought, since the laptop originally came with Windows Vista Business (there is a sticker on the bottom). If this is the case then going on the information given here I could use the key from the previous version (if the seller gives it to me) to downgrade.
The seller has replied that they didn't upgrade it, but that they installed it from a CD (!), leaving me a bit confused as to how they got Windows 10 for this machine (in System properties it reports that it is activated and the MS website also says it is genuine). I believe that you can download Windows 10 freely from Microsoft, but it won't activate without purchasing a Windows 10 licence key or upgrading an existing Win 7/8 installation.
I am a bit suspicious, but maybe this is unfounded since Windows is apparently activated. Is there a way I can check how Windows is licensed - ie. if the licence is a Win7/8 upgrade or an OEM/ retail version?
I should add that there is no option to restore the previous version of Windows in System Restore, and no hidden recovery partition on the hard drive.
Update 8 Nov 2015
slmgr.vbs -dlv, as given in Thomas Dickey's answer. I get the following information (I have blacked out some information I think might be sensitive, please let me know if I have shown anything I shouldn't have):
Have I understood this right? This is a volume licensed copy of Windows and it will stop working in 167 days unless it is reconnected to the company network where it was installed?