When you say format, do you mean you want to re-install windows or you want to format a secondary partition?
Also, what method did you use to try to format the drive?
To format your hard drive in Windows XP click on start. After that right click on My Computer and choose Manage.
Next click on Disk Management and you will see all hard disks and other drives mounted on your system.
Right click on the partition you want to format and click on Format.... After that follow the instructions to format your drive.
Warning: All data on the partition formatted will be lost.