First of all, you gotta know it will delete every file on your computer if you haven't saved it on a CD or put in on another partition.
I see you use Windows XP. Just put the XP CD in your CD drive, and reboot. And then just follow... It will ask you what you want to do, warn you that it's gonna delete everything, and then install XP. Then you will have to upgrade to XP service pack 2 (or SP3 but surfing without SP2 is pure suicide) the faster you can. And also install a firewall and an anti virus. I recommend you download it and put the programs on a USB flash drive.
If your computer doesnt boot on your CD drive, you'll have to enter setup and change the order of booting in a menu like "Boot devices priority". Just put your CD drive first, save and reboot.