It's easy Megan, just chill, lol!
Look at the back of the computer next to the Windows product key, or up on the side of the computer tower, for the white Service Tag.
In the tag write down what it states next to S/N.
This is the Serial Number.
(DO NOT post on here. Only give to an authorized HP representative)
Now contact HP Support, and ask for the Restoration Disk.
(HP bought out Compaq a long time ago)
In the USA the disk is free. The shipping and handling is not.
$17 for a 5 to 7 day ship, $24 for a 2 to 3 day ship.
In the US call 1-800-474-6836
24 hours a day, 7 days a week.http://www8.hp.com/us/en/contact-hp/phone-assist.html#section1
Outside the US use HP Support Worldwide,http://www8.hp.com/us/en/contact-hp/ww-contact-us.html
When you get the disk, turn your computer on, and put the disk in the optical drive's tray. (CD/DVD drive)
When asked choose Repair. (Non-Destructive Installation)
For additional questions please post in a Comment.