backup is: Backup extension is associated with Ad-aware reference files.You can try to open backup files with Ad-aware software.
Ad-aware software is generally used for saving your computer from trojans and similar harmful files. It is not enough to protect your computer from the viruses but it is good enough to avoid the risks which comes via internet explorer. The files which created by Ad-aware software are saved with backup extension.
Backup files may produced from windows7 restore function. If you need to get detailed information about the program, please check the following link.
Backup File format viewers or openers
Backup File related links.