If you're lucky, your laptop has a built-in recovery function, which might be able to put your system partition back into factory state. (While erasing all the files you have on that partition!). To do this, it needs to have a hidden partition with the recovery system and the recovery image.
Otherwise, you just have to do a standard Windows installation. Here are the basic steps:
- backup your files, if they are important
- put the installer DVD into the DVD drive
- boot on it. You can either go into BIOS and change the boot order settings, or you can press a key to select the DVD drive as boot device. Which key: it depends on your laptop model. Maybe it is displayed somewhere on the splash screen when you start your laptop.
- follow the instructions (one important decision to make is if you want to format the system partition, if yes, your files will disappear)
- after it got installed, you'll need to install the drivers as necessary. Maybe it's a good idea to download them now to a pendrive, if you can still start your system without crashing, because after the installation, you might not be able to use the internet until you install the network drivers.