What should i do with this computer?

So my moms boyfriend gave me a computer he got from a friend since he couldnt pay him for the job he did for him. Its a Lenovo that has a monitor but it doesn't have a tower, like the tower is already in the monitor. Its running a windows XP and it has the key in the back to activate it. I'm actually using it right now for this question. But my question is should i update it to a windows 7? Would it be all slow? The computer didn't come with the CD to make the computer all brand new again so i can make it my own, the computer owner still has his stuff in it but he doesn't need it. Where can i order the the brand new Windows XP restart cd? I already have the activation key for it so that won't be a problem.

You can just install windows 7 on it, the computer won't get slower unless its a computer from before Vista. If it is a really old computer. Throw it in the Trashbin. Outdated gear sucks.

As MS no longer supports XP check to see if your computer will support the Windows 7or 8 OS, if so you would probably want to upgrade and do a complete install of the Windows 8.1 OS.