Are you sure you actually did a format of the drive during Windows install, and didn't instead just install over top of Windows? Check to see if you have a Windows.old folder under your C: drive. If you do...that's where all of your old data is currently stored. You can safely move your personal documents/files from there to your actual profile directories (desktop, documents, etc.).
After that, you can run Disk Cleanup, and after it scans once, click the "Also remove system files" button and it will scan again, then give you the option to remove old Windows installations in the checkbox list.