I hate being the one that has to tell people what the guy likely really did, but it will save you cash down the road so its worth it.
First the overheating, "if" it was caused by dust buildup was likely fixed with a $5 can of compressed air, and a paperclip, you turn off the computer, remove power and battery (in case of a laptop) look under it for a fan opening, usually it is very clear as it will be the area shaped like a circle, sometimes it might be on the side if it is a side intake and you might need a flashlight to make sure of where it is, but it usually is easy to find. bend the paperclip where it has a straight part, and push that into the fan just enough to make the blades not spin, this is extremely important cause the air can will spin the fan blades many times what they were meant to spin and that will kill your fan bearings, a painful lesson i learned at a young age, actually got to see smoke coming from the fan.
Then look for the exit vents on your laptop, some have a fine screen over them some don't, if yours doesn't you can blow the air in from either where the fan intakes or from where the air goes out, if it does, you have to blow the air from where it comes out, and keep going until all the dust is blown out. takes less then 5 minutes. and you want to do this about every 3 months for the best life out of your laptop. also make sure it isn't on a surface that blocks the intake or outlet, i.e. don't set it on your lap without something solid under it, a blanket will kill it quick cause it cant breath. If you are using it hard like photo editing or gaming, look for a stand that has fans, just make sure that the stand blows the air the same way the laptop does, for instance you dont want a stand that pulls air from the bottom of the laptop if your laptop pulls air in from the bottom.
When it comes to a format, if you bought any kind of name brand laptop then it has a recovery install of windows, a simple google search of your make and model with the words recovery and how to will tell you how to use it and it is pretty much 100% automated, you don't need the serial because it already is in the computers recovery sector, the only thing that would mess that up is if you upgraded the hard drive and the person who did it did not know how to properly copy over your info, if they know what they are doing even a upgraded hard drive will still have the recovery section and your recovery will still work. This also can be found with google and is very easy to do.
the hardest thing to do is backup all of your personal data, but a external hard drive and knowing where your files are is all you need for that. it will take some time, but that is all.
in fact if you have not upgraded your laptops hard drive, your windows 7 recovery should still be there, so you should be able to go back to win 7 without the serial number
and i said if it was dust higher in the post because overheating can also be related to the fan in the laptop failing, if the air thing doesnt work, its your fan, that is something you can change if you have any level of technical skill (basically if you can build a boxed wooden or metal desk, you can do this with a how to from the internet) if that was the case replacement fans are usually pretty cheap, and all you need to do is google how to disassemble that model laptop, you probably can even find that info on youtube
computer repairs are far easier then techs want you to know, well except this tech that is.