Hello Gemma,
Firstly - welcome to
CompuForums. We provide our computer help for free, so we would highly appreciate it if you could help us. You can do this by answering other people's questions, or if you are not so technically-minded, post in our general lounge forum. Alternatively you can purchase a premium membership to help contribute to the rising running costs of this site. If you can't help us then we can't help you. This system allows us to continue to provide free computer support.
As Dr. Computer has pointed out, you can just run the recovery disks, or on some machines, the recovery partition. However - if you've ever stored sensitive data that you wouldn't share publically on the machine, consider running Boot 'n' Nuke on the drive. It can take a while, but it would make it pretty hard for most people to extract any sensitive data [e.g online banking passwords saved in your password, family photos, etc.].
It's free and you can get it from:
Darik's Boot and Nuke (Hard Drive Disk Wipe)