1.put the windows cd in.
2. reboot the computer
3. go to bios and set the cd-drive to be the 1st boot device
4. reboot
5. it asks for u to press any key if u want to boot from cd, so press something.
6. if u just want to reinstall windows then just install it on an existing partition, if u want to delete everything on the hdd and install windows on a clean hdd then delete the old partitions and make new ones and install windows on one of them (deleting the partition will remove all of the files on that partition)
7. follow instructions