It's not an etch-a-sketch!
You can use a number of mechanisms to remove and replace the existing installation of the operating system.
You can do a "clean" install of the OS either by using the built-in "recovery" partition installed by the manufacturer (obviously the only things that can be installed are those originally installed in this partition), or by using a valid OS installation CD.
However, you WILL also lose any installed applications (e.g. word processor, spreadsheet etc.), you will lose (almost) all your configuration settings and you MAY lose all your stored data - unless you copy all your data to a different disk or computer.
By "lose" I mean that you will need to re-install them from whatever installation media you have. No installation media, no application.
You will also have to re-install any SPs and updates from MS subsequent to the patch level of your installation media.
A good starting place for help and advice on what you can do would be your user manual, the Acer website and the MS website.
SD