So your problem is that you have an HDD that has an OS installed on it and you want to use that HDD in an other computer with an other OS? If so its really easy. Just put your new PC together with the HDD in it, put the DVD or USB stick into your PC (in wich you have your new OS, windows 7 pro) then go to BIOS (you need to press f12 or delete while your PC is booting up, but it can be something else too, it depends on your MoBo). In the BIOS you need to change the boot order so your PC will boot from the DVD/USB stick not from the HDD. If you done everything right, now ou should see the windows installer. From this point all you need to do is to format your HDD and follow the instructions on screen. If I wasn't clear enough here's a...