What type of computer do you have?
On one of my HP laptops, there's a built in recovery partition that's approximately that size, though it is not hidden. It may be a recovery partition.
I'm not too familiar with Acer. Did you check with the manufacturer?
Did you buy it brand new?
[JSH]John;1053445 said:Acer do create a recovery partition. If you ever have a major problem with windows, you can restore your laptop back to the way it was when you first got it.
I do not recommend removing this partition unless you have recovery disks. If you have recovery disks then you can remove it if you want, it just means it will take longer to reinstall Windows and all software that is preinstalled with your laptop.