Hi,

Thank you for reaching us.

We would like to know if you're using the Windows 7 DVD that you received with the PC or a retail copy that you bought separately.

If it is a Dell copy that's being used on a Dell computer, It doesn't require a product key to be manually entered by the customers, Also its a single license copy which can be used only on one PC.

Thank you.

First since you are trying to remove a virus/malware don't settle for a format. You want to wipe the drive instead. See here:

http://dellwindowsreinstallationguide.com/cleaning-up-a-drive-format-vs-secure-wipe-ssd-and-hdd/

Next Windows 10 is going to be released on the 31st of July 2015. Microsoft's representative Gabriel Abu has made it known that there will be no cutoff for Windows 10 Insiders from reaching the Windows 10 RTM. I would advise you to install the Windows 10 Insider Preview and use it as a jumping stone to Windows 10 RTM opposed to fighting with your Windows 7 product activation issues and then taking the same jump to Windows 10 RTM (from the free upgrade from Windows 7):

http://dellwindowsreinstallationguide.com/download-microsoft-windows-and-office/download-microsoft-windows/download-windows-10-technical-preview-iso/