Steam - Missing App Configuration

Today I was trying to install a backup into Steam, and I got this error message:

An error occurred while installing Grand Theft Auto IV (missing app configuration)

This got me scratching my head until I went onto Bing and did a search.  I found this helpful post:

http://forums.steampowered.com/forums/showthread.php?p=33797294

Turns out all I had to do was delete the "Steam/appcache/appinfo.vdf" file.  I did so, and the backup rolled right into place.

If you're having trouble finding the file, the follow these steps:

1. Right-click your Steam icon (on your desktop or whereever it may be).
2. Click on Properties.
3. Click on the "Open File Location" button.
4. An Explorer window should open.  Double-click on the "appcache" folder.
5. Find the "appinfo.vdf" file (probably at the bottom of the list)
6. Right-click the file, click "rename", and name it something else (like "appinfo.vdf.old")

I hope that helps.