The hard disk is generating error messages or is otherwise behaving as if it has become seriously corrupted. An existing, working hard disk may be acting as if it had been wiped clean, or had been never formatted. Strange error messages or very large quantities of files may be corrupted or wiped out. Note that small numbers of lost clusters or other minor file system corruption are often a result of more benign situations and are discussed here instead. In many cases it is not the hard disk itself but rather an external factor that is causing the problem.
Many viruses take hard action against the file system.
It is a relatively simple thing for a virus to change the disk's major structures so that it appears empty or invalid. Make sure that you are using the correct type of disk utilities. Using a disk utility not designed for Windows 95 on a Windows 95 system, or using one not aware of FAT32 on a FAT32 volume, can cause large amounts of corruption on a disk that is otherwise functioning properly. If you have just installed the disk or made changes to the inside of the system, try this troubleshooting section that focuses on common installation problems . If you have just moved a hard disk from an existing system where it used to work, the problem may be related to a change in how the BIOS references the disk.