It would help to know which Aspire One model number you have. However, you can try to recover the original factory settings from a hidden partition on the hard drive if it is still intact. Just follow the instructions in this video. https://www.youtube.com/watch?v=H2xoMEPfiPQ
OK, your system probably has Windows XP installed. First, shut down all power to the machine. Then re-start. At the ACER logo splash screen, tap F2 to enter the BIOS menu. In the BOOT dropdown menu, make sure D2D recovery option setting is enabled. Then F10 to exit the BIOS menu and save the BIOS setting.
Then shut down all power to the machine again. Then follow this youtube video using the Alt+F10 startup recovery method. https://www.youtube.com/watch?v=H2xoMEPfiPQ There may be variations in the recovery options you see due to the differences between Windows XP and Windows 7.
If the hidden ACER recovery partition on the hard drive is still intact, the Alt+F10 startup recovery method should return the system to a factory-fresh state. Good luck.