Question: How do I reformat a hard disk? (My unit is a Pentium 4 with 40 GB hard disk)
Answer: If it's a second hard drive and your computer recognizes it you just right click My Computer and pick Manage. From there you would open Disk Management, find the second hard drive, right click it and pick Format. Then pick if you want FAT or NTFS. I recommend NTFS.
If you are going to format your main hard drive (C drive) you can do so by booting to your Windows XP CD. Make sure you BIOS is set to boot from CD before hard drive or else it will keep booting into Windows.
After you boot to the CD, Windows will load files and eventually get to a point where it asks where you want to install Windows. From there it will display your hard drive configuration and give you options to leave it as is or format it. When you select format it will ask if you want to do a regular format or quick format. If you aren't having problems with your hard drive you can get away with a quick format. A regular format takes longer but checks the disk for errors. For a 40 GB drive it won't be too long so a regular format will be fine too. After it formats it will copy files to the drive and start the Windows installation.
Keep in mind that formatting the drive erases EVERYTHING so back up your files first!