Which you follow will normally be determined by what CDs (if any) were provided with your computer. If you have neither of these available, you could try contacting the machine's manufacturer or supplier.
In some cases, a manufacturer may have placed a copy of the Windows installation CD
contents in a folder on your machine's hard disk, typically located at
c:\windows\options\cabs. If you are in this situation and are
able to transfer the conents of this folder to a CD, then this could provide you with a
suitable re-installation source, although you might also need a separate Windows boot CD
or floppy disk. If you do not have such a disk, you may be able to create one via http://www.bootdisk.com.