Cannot recover OS to used laptop

Look on EBay or similar -- there are usually people selling OEM Dell install DVDs for a few dollars.

Neither Dell nor Microsoft ships Windows 7 media any longer.

Either that, or if you’ll be using this system for more than 18 months, download the Windows 10 ISO from Microsoft (which does not require a product key) and purchase a license to activate it after you install it, since Windows 7 will no longer be receiving updates as of January 2020.

If you’re determined to use Windows 7 though, do yourself a massive favor and manually download certain key updates beforehand so you can install them immediately, otherwise Windows Update could spend hours just trying to figure out what to start downloading, and then several more hours and restarts to get everything. Below are the updates I would recommend downloading beforehand and then installing offline before you even start in on Windows Update.  The updates must be installed in the sequence listed below, and this will mean your first Windows Update experience won't be excruciatingly painful.  I would post direct links, but including multiple links in a single post tends to get the post held for moderator approval, which can take a while to come, so you can just Google the names/KB numbers below to find the download pages from Microsoft:

- Windows 7 Service Pack 1 (unless you'll be using install media that has it integrated)

- KB3020369: April 2015 Servicing Stack Update (required to install subsequent updates)

- KB3125574: Windows 7 Convenience Update (takes you from SP1's February 2011 state all the way to April 2016 in one big package rather than hundreds of individual updates)

- KB3172605: July 2016 Update Rollup (includes the fix for very long Windows Update scan times, so after installing this, you may still have a few dozen updates, but at least you won't be waiting potentially hours for them to even start downloading).

Thanks for Your help! Just one more question: What happens if I buy the recovery disk on e-Bay and maybe somebody somewhere already installed it. Will it work anyway on my laptop as well, or is there a possibility, that it will lose its activation on my machine due to being previously activated on another?

Thanks

Windows installation discs aren’t unique for specific activation keys, nor do OEM installation discs come with product keys like retail discs, so there’s no risk to using a physical disc that was previously used elsewhere — but as ejn63 says, you may need to have the product key that would have been included with your own system (assuming it was ordered with Win7 Pro), since that’s normally how you get Windows to activate. But even if you don’t have it, if you would really prefer not to purchase a Windows 10 license and can find a Dell Win7 Pro install disc for a low cost, you may want to roll the dice and see if it happens to activate automatically on your system, since OEM install discs can sometimes do that when used with systems made by that OEM.