First off, the other answer is BS since you say clearly that you don't have an install disk. You might even have some issues trying to sell it without a Windows disk. Anyway, you can't format the drive as you suspect because it won't have an OS. Go into control panel and uninstall everything you want to uninstall. Most programs you could actually leave, I would think. Then open up your file explorer (right click the start button and click on "explore"). The only thing that involves privacy is your pictures and documents. Make sure you have saved what you want elsewhere. Go into those folders on the left panel. On the right panel click the first file, scroll to the bottom file, hold down SHIFT and click on it, which will select everything in between. Then hold down SHIFT and hit delete (on the keyboard) - that will delete the files instead of sending them to the recycle bin. Say yes to anything that asks if you are sure. You can do this in any folder you like but stay out of the whole Windows folder. Then empty the recycle bin, go to RUN off the start button and type cleanmgr, which will run disk cleanup. Check off everything and run it. That could take a couple of hours...... That's about what you can do, since you can't just format the drive.