Backup! Backup Backup!
In some (Business, Ultimate, Enterprise) versions of Vista, you can easily restore a file from, so called, shadow copies... See here: Previous versions of files
Unfortunately, it's not so easily available in Home versions... Maybe somebody have a better idea, but that program might be of help: ShadowExplorer
- if a System Restore, for the disk that file is located on, is turned on, you might be probably able to get that file back...
Then, make a backup... Good luck!