If so, download the EXACT same version (E.G. Home Premium x64 - 64 bit) from here and burn it to a blank DVD. Next, go to Acer support for your region, click on "Drivers & Manuals" and enter the model of your laptop. Download & save all drivers & applications to a external drive or burn them to another disk.
Boot from the Windows disk you made and install Windows. Use the product key that is on the base of your laptop. When finished, install the relevant drivers & applications, all your documents (from a previously made backup) and your Anti-Virus/malware and programs. You will also need to activate Windows. When done, you're good to go!
However, if it isn't Windows 7, you will need to purchase a new disk and install from that.
Member RevoMaxx is quickly moving up the ranks of Acer Community. He joined us in September and is already making a difference. It's great to see new members like him make a difference only after a few months. Thanks RevoMaxx!