Let's spell this out;
1] Do you have the GENUINE Windows Setup CD (XP SP1 or later SP) or DVD (Vista or Win 7) to reinstall Windows? If not you cannot reinstall Windows.
2] Do you have the DRIVERS CD to install all the drivers for this Windows OS?
3] If this is Windows XP, (we still don't know which Windows), and the hard drive is a SATA drive, do you have the SATA drivers?
If you have a genuine Windows CD or DVD then you can use that, (insert the CD or DVD, reboot the computer) to delete the existing partition, format the drive, then install Windows. If this Windows is XP and if the drive is SATA, then you install the SATA drivers during this setup process. After the setup is complete, you then boot up into Windows and install all the drivers.
4] Does this Click Free backup drive hold just your data, (Personal Files), or does it hold an IMAGE of your Windows installation?
If it holds a complete image of your Windows OS then follow the guidance in it's manual or other documentation to restore that image to the hard drive.
Expect problems. There nearly always are problems. So make sure you know what you are doing and you have a plan.
Mark