Easiest way, assuming you're using XP (taken from the Microsoft site because I'm lazy):
1. Insert the Windows XP CD-ROM into your CD-ROM drive or DVD-ROM drive, or insert the first Windows XP Setup disk into the floppy disk drive, and then restart the computer.
2. If you are starting the computer from the Windows XP CD-ROM, select any options that are required to start the computer from the CD-ROM drive if you are prompted to do this. (Important - if it says something like "Press enter to boot from CD-Rom, hit enter...)
3. At the Welcome to Setup page, press ENTER.
4. Press F8 to accept the Windows XP Licensing Agreement.
5. If an existing Windows XP installation is detected, you are prompted to repair it. To bypass the repair (which you want to do), press ESC.
6. All the existing partitions and the unpartitioned spaces are listed for each physical hard disk. Use the ARROW keys to select the partition or the unpartitioned space where you want to create a new partition. Press D to delete an existing partition, or press C to create a new partition by using unpartitioned space. If you press D to delete an existing partition, you must then press L (or press ENTER, and then press L if it is the System partition) to confirm that you want to delete the partition. Repeat this step for each of the existing partitions that you want to use for the new partition. When all the partitions are deleted, select the remaining unpartitioned space, and then press C to create the new partition.
7. Type the size in megabytes (MB) that you want to use for the new partition, and then press ENTER, or just press ENTER to create the partition with the maximum size.
8. Repeat Steps 4 and 5 to create additional partitions if you want them.
9. If you want to install Windows XP, use the ARROW keys to select the partition where you want to install Windows XP, and then press ENTER. If you do not want to format the partition and install Windows XP, press F3 two times to quit the Windows Setup program, and then do not follow the remaining steps. In this case, you must use a different utility to format the partition.
11. Select the format option that you want to use for the partition, and then press ENTER. You have the following options:
• Format the partition by using the NTFS file system (Quick)
• Format the partition by using the FAT file system (Quick)
• Format the partition by using the NTFS file system - CHOOSE THIS ONE
• Format the partition by using the FAT file system
• Leave the current file system intact (no changes)
then it'll reboot by itself, so take the CD out of the drive and let Windows reinstall. tah dah.