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) [
or just transfer all files to the other drives , they wont get formatted ]
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
a. restart your computer
b. Go to your Bios setup (keep pressing delete or F2 or F8 /differs for different companies/)
c. Try to find the Boot -->Boot device priority option
d. You should find choices like cd drive, Floppy drive , Hard disk etc...... By default your Hard disk would be chosen ..... change it to your DVD/CD drive
f. Save and Exit
Now the computer will start booting from your CD
NOTE : its not half as complicated as it sounds .....Give it a try
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