Problems with Acer computer

Bought a used Acer computer yesterday. They said it worked.
When I brought it home. Apparently they tried to change it from Vista to XP.
Well it didn’t work apparently and it is asking for a activation key or something along that.
Is there a way to restore the computer so it has Vista back on it?
I don’t have the cds or anything. I can’t get to the main screen because a screen keeps popping up asking for the activation key or to activate XP.
Answer #1
Give a screenshot.
Answer #2
What windows xp are you currently running?
Answer #3
amandabee replied: Bought a used Acer computer yesterday. They said it worked.
When I brought it home. Apparently they tried to change it from Vista to XP.
You should’ve checked it was working at the time of purchase
amandabee replied:
Is there a way to restore the computer so it has Vista back on it?
I don't have the cds or anything. I can't get to the main screen because a screen keeps popping up asking for the activation key or to activate XP.
You can use the Search button here, and download Vista, but if your computer will run that why not go for Windows 7 ? Just format the harddrive and install it.
If I bought a second hand computer, the first thing I would do is format it, probably full of malware.