the only thing left is to buy new windows and install again?
Nope..... you don't.
IF you had Windows 10 installed and activated on the computer, you can reinstall it anytime for free by downloading the installation media from here
https://www.microsoft.com/en-us/sof...459594)(TnL5HPStwNw-WaTVja5C7z2Vs7q82apCPg)()
Match the ISO that you download to the version that you have installed.
Choose Windows 10, the first item in the first drop down (not single language at the bottom) * see note at bottom.
Next choose your language and your bit version (32 or 64 bit to match your system architecture or in the case of an upgrade, to match your currently installed version of Windows).
That will provide an ISO that will boot, upgrade, repair (or clean install) either or both Windows 10 Pro and Home.
Within Windows 10 you can just double click the ISO file to mount it as a virtual drive and run setup.exe from that virtual drive to perform the in-place / upgrade / repair.
*NOTE: In some cases, we've learned that some people have the "Single Language" version installed. In which case you would need that download. It's important that you match the ISO version that you download with your installation.
Once you have downloaded the ISO you can use ImgBurn to burn it to a DVD
http://imgburn.com/index.php?act=download
OR
Rufus to burn it to a USB ThumbDrive
http://rufus.akeo.ie/
Boot from it and install