If restore CD's came with it, use those, i.e. boot directly to the DVD drive. If not, there is a restore partition on it. Usually you can access this by hitting fn+f5 (or whatever it is for acer's, as it is different depending on the brand).
If restore CD's came with it, use those, i.e. boot directly to the DVD drive. If not, there is a restore partition on it. Usually you can access this by hitting fn+f5 (or whatever it is for acer's, as it is different depending on the brand).