Which operating system is currently installed?

OK-- Win7 Pro
There will be a recovery partition on that laptop. You can use that to reset the computer to the factory defaults. In other words, it will be like it was when you bought it off the shelf, complete with all the installed bloatware.

I haven't used a recovery installation, but you want to be sure that you format the drive if you have sensitive files on it that you don't want anyone else to see.

If the recovery option does not offer to format the drive, you should do that manually before you begin.

To access the recover operation usually requires that you press a specific key during boot time, that is, before Windows loads. This window of time can range anywhere from three seconds on up depending on if you've changed the settings.
The key that you have to press also varies by manufacturer. On my laptop, it is F2, but that might not be the case on yours.
As the computer boots, you should see options to press various keys to do various things. If you don't react fast enough, then reboot the machine and hit the right key.

If for some reason the recovery partition has be deleted, then you will have re-install Windows from scratch. You will also have to know your product key in order to activate Windows.

Also, if you have important data kept on that machine, you should back up those files before you begin.
Good luck and I hope I haven't forgotten anything.
Richard