Your computer may be infected with a boot-sector virus such as the AntiCMOS.A virus.
Your computer is running an anti-virus program.
Your computer is using hard disk management software (such as Disk Manager, EZ-Drive, or DrivePro) for hard disk geometry translation. These tools provide support for large hard disks (more than 1024 cylinders) when your computer's Basic Input/Output System (BIOS) does not support large hard disks.
Windows Setup was unable to copy the system files to the hard disk.
Security software has disabled access to drive C.