First of all check your manual it should state which keys you need to press to restore your system.
If it came with the OS already installed it may already have a partition on your hard drive with the recovery files on it or you will have received a recovery cd with it.
On some you just have to reboot the system and select the right f key others you put the recovery disc in and boot the computer up if you are using a recovery disc make sure that your boot system is set to boot from cd first
