Did you create recovery disks as recommended when you setup your system originally? If you have those disks, put disk 1 in and start your laptop - it'll guide you through the recovery process.
If you did not make the disks (or have since lost them) your pretty much limited to locating an install disk for your version of windows (a local PC shop may have an install disk - may be a small fee for use, depending on the shop's policy) and doing a complete install - you'll use the product key that goes with the laptop for validation