You can attempt to restore your computer to "factory settings" by performing the actions below.
0. Buy a disk that will install your operating system.
The following directions are to be performed from when your computer is off.
1. Press the power button on your computer.
2. Immediately after the computer is on, repeatedly press F12, until you are in the Boot Menu.
2a. If the computer boots as normal, turn off the computer, and repeat steps 1-2, replacing the action of pressing F12 with pressing another F-key. You can also attempt using ESC.
3. Either way, you should be in the Boot Menu. Press the button next to your primary CD-ROM.
4. Place an operating system installation disk on the CD-ROM tray.
5. Press the button next to your primary CD-ROM.
6. Use the arrow keys to select the option for booting from your CD-ROM.
7. Press Enter.
8. Press a key.
9. Follow the directions to install the operating system.