I've been fighting this problem for months. Around 2008-2009, I purchased a refurb'd Studio 1735 laptop directly from Dell. It came with Vista Home Premium OEMAct factory installed. There was some special deal at the time that included a Windows 7 upgrade. When I received the product, it came with the Vista DVD AND a Dell OEM DVD for Windows-7 Ultimate x64 (not an upgrade DVD, but the whole package). Cool!! I turned on the laptop, brought it up, tested it out to verify everything worked, shut it down, and rebooted the Win-7 DVD and installed it. No activation problems, self-activated. Voila... very great laptop. Been very pleased with its performance. Time passes....
In early 2015, the Motherboard died. Bummer. Hmmm.... Dell seems to want a lot of money for a replacement. Ended up buying the IDENTICAL model laptop from eBay for half the price of a motherboard from Dell. It had a bad LCD, but all else was good and a lot of spare parts to boot. Repaired my Laptop, brought the system up... and 3 days later got a message that I need to Activate Windows. WHAT!! After some research, it seems that Microsoft deems that replacement of your motherboard is considered installing Windows on another PC, which of course, is RIDICULOUS!!! The unit that I bought from eBay also had Vista Home Premium OEMAct factory installed. Still has the OEM Product Key sticker on the empty case. (my original system does as well)
I went the route of calling Microsoft which was, and always seems to be, an exercise in futility. They basically told me that since this was an OEM product, I needed to contact the OEM (that's YOU, Dell) for help. But the "agent" did offer to sell me a key for $99... my, how helpful. I hung up and installed Linux Mint the next day. (from which I write this post)
It seems that Dell and Microsoft love placing you between a rock and a hard spot.
I would still like to go back to Windows. Linux has a lot of nice stuff and I'm learning something new, but I find there are things I could do with Windows... that I just can't do in Linux.
So, Dell, can someone there step up to the plate and help me activate my Windows-7 product?