Did that computer come with CDs for Windows and other software originally installed on it? If so, put the Windows CD (may be labeled Recovery ...) in the CD drive and restart the computer. It should boot from the CD and display a screen with options to format the hard drive, create paritions on the hard drive, install Windows, etc.
Formatting erases everthing from the hard drive. The objective here is to restore the system to state it was in when new; however, some extra software that was originally installed may not need to be reinstalled. For example, if the computer originally had a shortcut on the desktop to install AOL or an trial version of an anti-virus program, those things do not need to be reinstalled.
If you do not have the original CDs, then this approach is not an option. Then, you're stuck with trying to cleanup the mess she created. If you have the CDs, I believe a fresh installation of Windows is going to be, by far, the cleanest solution here.