Many different causes can lead to a corrupt file, from a system crash to a virus.
Unfortunately, it's hard to always pinpoint the exact cause, and it's not always possible to remedy the problem. If you have a diagnostic software utility such as Norton Utilities, it will analyze your system and look for any hard drive problems. If the program finds a problem, it will ask you if you want it to fix it, and then it will do so if it can. You can also try running the DOS utility ScanDisk, which you should find in your Accessories folder under System Tools. This program also will look for problems on your computer.