Any files that have been overwritten, which means the first ~5 GB on the disk, are gone. You can attempt to use testdisk/photorec to scan the drive to see what's recoverable, but it's going to find a LOT of trash that you'll have to sort through manually. Afterwards, you'll have to format the disk and reinstall Windows from scratch, and then re-load whatever files you managed to recover back onto it.
It's going to be a LONG and painstaking process. If you have a backup then use it, otherwise good luck.