If you follow the instructions as normal to install Windows (I wouldn't do this unattended), about 4-5 screens into the installation it detects you already have Windows installed and gives you the opportunity to repair it. What this does is delete the Windows files and copy them back off the CD. Your User Accounts will be safe, your installed applications will be safe. It's worth a try, but be prepared to reformat and install fresh if it goes pear-shaped!