1) System Restore will not help
2) It is NOT true that deleted files are immediately overwritten - in fact, I've never even heard of that before. The only thing that happens when you delete a file is the pointer in the master file table is deleted so the system does not see the file and assumes the space the file occupies is now available, but the file remains as it was until something does indeed overwrite it.
3) I agree you should try Recuva