Simply boot to your recovery disc and follow the on screen prompts.
This will recover your system to factory default settings.

Some OEM PC's that did not come with recovery disks have a hidden partition on the original hard drive you can access to get back to factory default. You usually access it during start-up by hitting a key that will be listed at the bottom of the screen, like F2 or F11 something like that.

All my HP computers have the hidden recovery partition which is activated by pressing F10 during bootup.

If you're unable to find the hidden recovery partition and activate it, post back your complete computer make and model #, we'll see if we can help Google up the specifics.