you cant format a harddrive and have all your data intact,a format erases the hdd of all data .
having said that i think i now understant what you meant .you have you drive partition into 4 parts,windows on c:\ and data on othere 3 correct,if so then just boot to your windows cd and you will get option to format c:/ and install windows to it ,all 4 partition will show in the list to chose from ,make sure you chose correct one ,they will belisted by size so wright dow the drive letters and size to make sure you are formatting correct one
1 restart your computer and go to setup usually by pressing the F2 or delete key.
2 Once you get into setup look for the boot menu and
make sure you set it to boot from cd first followed by your hard drive.
3 Put the Windows xp disk into your cd.
4 Now save your settings and exit setup.
5 While your computer is booting you will see a message that says "press any key to boot from cd" press any key.
6 When the welcome to setup screen appears press enter and then press F8 to accept the Microsoft licence agreement.
7 You will be prompted to repair an insallation press the escape key.
8 Now select the partition that you want to reformat and
press the D key to delete it you will be asked to confirm that you want to delete the partition.
9 Now press C to create a brand new partition you will
be asked what size you want the partition to be in mega bytes.
If you just press enter then the partition will be the maximum size that you can have.
This is perfectly ok if you don`t want to create multiple partitions.
10 You will now be asked to format the partition select the ntfs file sytem and do a full format.
11 Once the format is complete setup will continue.
Your computer will restart during the remaining setup again you will
be asked to press any key to boot from cd DO NOT PRESS ANYTHING. and setup will continue.
Once the setup is complete and you are back in Windows remove the Windows cd from your cd drive.