First of all, sorry for late response. Was dead busy.
Anyway, I will advise you to first backup your current data on another
partition or removable media, if you can. As you said that your data is lost
then I will advise you to search for the same on C drive or Boot Partition
where window is installed. If you had stored the data in My Documents or in a
folder on Desktop then try searching for the same in Documents and Settings
folder. You will surely get it in Documents and Settings\User Name sub-folder.
Next, if you have a bootable Windows XP CD then restart your PC and change
the first boot device to CDROM and install the Windows. In the installation
process choose C Drive as installation partition and when asked then format
the drive and continue the setup. This will remove all the previous version
of Windows installed on C drive and will free up the disk space.
If you have installed 2 copies of windows, one of C and another on D drive
then do as said above and after you installed the windows successfully. Move
your data from D drive to another partition like C or E and then format the D
drive as well to free up the disk space from d drive.
Hope this help, let us know!