Repair shop put illegal OS on my desktop- I didn't ask for that...Am I stuck forever with pirated OS?

Contact Microsoft with the information of the repair store and explain what happened. I would be very cautious about putting personal info onto that computer, especially financial stuff. Maybe give it a once over with this or some other software to see if anything pops up.

I can't really help with installing a legit copy of Windows because I've never done it.

I would explain to them what they did vs what you wanted them to do. Ask if they can just clone what you needed instead of give you hacked software without your informed consent. You should probably use the phrase informed consent too so they get a lawyer vibe and do what you want.

Other than that you could just do the clone yourself. Can't be that difficult.

Do you still have access to the old hard drive? Will it not fit in your new computer? Or you could image the old one and put it on the new disk. If you need an OS to do this I'd use Linux Ubuntu or Mint booted up on a USB stick or CD.

dd is the command you are looking for in Linux.

If the two hard drives are different sizes you'll probably need to adjust the partitions afterwards.

Let Microsoft know about it. If you have a prebuilt computer from Dell, HP, Lenovo, etc. that is a bit older there is usuaully a sticker somewhere on the case that has your Windows key.

If your able to find it, you could shrink your main partition (if you have room), add a new partition, put all the data you want over there, reinstall Windows with a proper key (making sure not to erase the partition with your documents your saving), transfer the files over to the main partition, delete the temporary partition, expand your new partition, and your good to go.

There’s a lot of gray area here. Is there a CoA sticker on your machine denoting the license? Where did you get Windows from in the first place? I assume you’re using Windows 7. If you have an OEM then it’s tied to your hardware and not legally transferable... though it could probably be done. If it’s a retail version, you can. In either case, though, you need the key.

https://www.makeuseof.com/tag/how-to-make-your-windows-7-pc-genuine-again-after-a-hardware-upgrade/

In any case, what they did is illegitimate if you’re describing this correctly. Sometimes the license doesn’t transfer or you simply cannot clone your existing installation onto new hardware.