A client of mine had his computer crash. Luckily, I had suggested he subscribe to Carbonite (an online backup service) and he was able to recover all his ACT files and all his other data.
When we attempted to restore his ACT data we ran into an issue; a file was corrupted. I suspect he had a virus. Because ACT is made up of more than one file I urge you to do a regular backup of ACT so you have the option of restoring ACT from a backup, in case you run into issues with the files. If you save the backup on your hard drive, it will be backed up by Carbonite.
To create a backup, select File | Backup on the ACT menu.