Before you do anything, is the data on the computer important, and is there a recent backup?
If not, get an external hard drive. Download Ubuntu or Linux Mint on another computer, and burn a startup DVD. Start up the laptop with the Linux DVD and copy off all your important data. You can also use the Linux DVD to make an image of the original HDD.
Your Acer may have an OS restore partition. If it does, try using that to restore the original OS.
If it doesn't, Microsoft has Windows 7, Windows 8, repair disks that you can download. Google for the appropriate one, and use it to restore your original OS
or
Windows 10 is available for free download from Microsoft too. If you want to go forward, download that ISO, burn it to DVD, and use that.
Whatever you do, make sure your data is safely copied off the disk before you proceed. I've seen twice where a Microsoft spontaneous downgrade (back to Win 7 without user selecting that) has wiped out all user data except the user name and the password. ALL DATA!
Your other option is to just go to Linux. I use Ubuntu on all my computers. I've changed a few people over to Linux Mint and they are happy. It's much faster to set up a computer with Linux than to set up Windows. 20 minutes vs. 4 hours. I have no pestering pop ups. No malware worries. No yearly antivirus costs. No headaches. None.