I guess u r asking for formatting and reinstalling the OS.
Things you Need :
Windows XP Geniune CD
Flash Drive (Incase you need data Backup)
Motherboard CD
Other Driver CD's (eg., Printer Drivers, Webcam's etc)
Procedure :
1. Log in to ur current OS and back up any files you need
2. Boot your System with Windows XP CD
3. It will load all the files, and ask you which partition to install the OS., Just Select the partition and press enter.,
4. Windows will do other jobs
5. After giving CD Key, Selecting Time Zone, etc Winodows will start installing the OS.
6. After Installing the OS, Install the Mobo drivers and other required drivers
7. Install the other softwares
Cheers