Not sure how old the ACER is, but they usually have a hidden partition with a full system recovery option on it. Hitting F11 during bootup usually gets you to the recovery. If not, boot into Windows normally, and there will be an option under the All Programs (probably under another folder, so just open a couple to see if it's in there; start with anything called PC Tools or even the ACER name itself).
Just choose the option to recover to factory without backing up data and you should be good. Just beware that if the hard drive is on the downside of its normal life it may go very slowly or even fail.
(If it won't do an auto recovery, you may have to burn a set of discs from the built-in recovery partition first.)