Operating System Hard Drive Wipe Out

I recently downloaded a free software program on the Internet that was intended to clean the hard drive on my computer so I could resell my computer without compromising personal data. After download, I began using the software to clean files on my computer, and as I was doing this, a Warning prompt appeared, telling me that the files selected could not be cleaned while they were in use. Therefore, I clicked Ok, but after I did this, my computer began freezing up on me. So, I decided to restart my computer. When I did, the computer screen went black, and I began getting a message telling me to hit Ctrl Alt Delete to restart. When I hit Ctrl Alt Delete, it simply takes me back to that same screen. I have tried to restore my computer, but the system screen does not have a Restore option. I can't even get into the Command screen using keystrokes. I have tried the usual keystroke for Restore, and it does not work. How can I get back into my computer so I can resell it? It is Windows XP and an HP Pavilion, 2002 computer. I do not have an Operating System disc.