does windows 7 upgrade version work on a build PC with no OS installed. i heard that you need a previous version of windows like xp or vista to use it. but i have a PC that have xp installed but not working. can i use the product key of that computer on the new build PC to activate windows 7
If it's an OEM version then no you can't OEM versions are locked in to the pc they were installed on and are not transferable. In any case you can't just use the key for the upgrade, it requires the previous OS to be installed and activated.
"does windows 7 upgrade version work on a build PC with no OS installed"
Yes it does but it does require you to own a previous qualifying OS that is been removed from the pc it was installed on.