You need to reinstall the OS (in your case Windows XP).
Anyway, if you have at least 1GB of RAM, you can install Windows 7 as well.
For any operating system, you need to have a bootable CD / DVD (if your laptop has an optical drive) or a bootable USB flash drive
Here you will find how to install windows 7
and also, here you can learn how to make a bootable usb
no matter what operating system you want to use.