To wipe all data and make sure nothing can be retrieved, you will have to format the drive several times - I believe its seven times if you want to be absolutely certain that its wiped clean, and that no one can retrieve any data from the drive.
Have a look for hard drive erasers on download.com or tucows.com.
If you wish to just reformat and reinstall, lets assume its Windows XP that you are reinstalling since you didn't say:
1. Put Windows XP into your CD-ROM, reboot your machine.
If your machine doesn't boot from the CD-ROM drive, you need to change some settings in BIOS. Reset the machine again, tap DELETE or F1 to get into your BIOS setup. Change your boot priority to include your CD-ROM drive before the hard disk. Save and reboot machine into Windows XP CD-ROM.
2. When the installation disk finishes booting, follow the instructions from there to reformat your hard disk and reinstall windows.
Have a look for hard drive erasers on download.com or tucows.com.
If you wish to just reformat and reinstall, lets assume its Windows XP that you are reinstalling since you didn't say:
1. Put Windows XP into your CD-ROM, reboot your machine.
If your machine doesn't boot from the CD-ROM drive, you need to change some settings in BIOS. Reset the machine again, tap DELETE or F1 to get into your BIOS setup. Change your boot priority to include your CD-ROM drive before the hard disk. Save and reboot machine into Windows XP CD-ROM.
2. When the installation disk finishes booting, follow the instructions from there to reformat your hard disk and reinstall windows.
