See: http://support.hp.com/us-en/drivers/selfservice/HP-TouchSmart-300-Desktop-PC-series/5146287/model/51...
Order Recovery Media-CD/DVD/USB (1)
USB-Windows 7 64-bit Recovery Kit
System Recovery Kit for Consumer Notebook
Order Media
Click that hyperlink to order a copy of the originally-installed software.
P.S. How old is the disk-drive? Is it "out-of-warranty"? Probably, for many years, since the warranty probably was just 2 years. Since you will be reinstalling Windows, I recommend that you purchase a brand-new disk-drive, and install Windows to it. Then, you can attach the current disk-drive as a "slave" disk-drive, and copy all your personal files from "old" to "new".