if a pc did not come with installation cd's, then there might be a hidden partition, that would take pc back to out of box experience or factory settings.
You might also contact acer and request a replacement installation cd.
any downloads available on the net are most likely malware ridden.
a serial number is not needed, a Product key is what is needed and it should be on a tag somewhere on the body of the pc.
if you can find a friend with a cd of the exact type of OS on the pc then you can use that cd and your product key.