It's really frustrating, and it's happened to all of us: Accidentally deleting a file that you need. The good news is that if you act fast you might be able to recover the file.
When files are deleted they aren't actually erased, they're just hidden until another file overwrites them.
FreeUndelete scans your drive and finds these files. When I ran it, I found thousands of files that had been deleted, but not overwritten.
Once downloaded, just click the drive you wish to scan, then click Scan. You will be presented with a list of deleted files. Once you find the files you wish to retrieve, just click Undelete.
If your computer only has a C: drive, FreeUndelete suggests that you use a USB flash drive as a destination for your recovered files. Otherwise you might overwrite some deleted files as you recover other deleted files.
Of course, you can sidestep the entire worry with an always-on, online backup service like my advertiser Carbonite. Recovering a deleted file is as easy as restoring it from your online backup.