Insert the disk and restart the computer.

Press any key to boot from cd

Choose install.

Do the agreement.

Now it's your choice, If you want to just repair it, it will do so without losing any files and folders(Hopefully. This is why we backup any files and folders we cannot afford to lose)

Follow the instructions.

Remember to go to Dell's website and download and install the drivers in this order.

Desktop System Software or Notebook System Software
Chipset or SMBus driver
Processor Driver (if applicable)
Audio (sound)
Input Drivers (keyboard/mouse)
Network Card

Also you'll have to get all your Windows Updates again.