The CIH (a.k.a. Chernobyl or Spacefiller) is one of the most fierces virus available. On its activation date (April 26th) it tries to erase your computer’s BIOS and also your hard disk.
To recover your hard disk, you will need to use a data recovery software. From all softwares we tested, the best one is the Fix-cih, which is free and can be downloaded at http://www.grc.com/files/fix-cih.exe. This software is small and really efficient. You will need to create a bootable floppy and copy this program to it, and then boot the infected computer from this floppy. Format this floppy from a computer without virus (of course) and using at least Windows 98 (if you format it using DOS or Windows 95, it won’t recognize FAT32 partitions and you probably won’t be able to recover your hard disk). Run the software and wait. It can take a couple of hours recovering your data, specially with you have a large hard disk.
After recovering the hard disk, you will need to run an antivirus software to remove the virus, that will still be stored on your hard disk. We recommend you to download and run cleancih, which can be downloaded from http://www.pspl.com/download/cleancih.exe. This is a 20 KB DOS software, so you can copy it to your bootable floppy and run it after booting from a floppy. Don’t try to boot your from your hard disk, because it is infected and you won’t be able to remove the virus.
To boot your PC from a floppy disk, you need to enter setup (pressing Del key during the memory count that occurs when you turn your PC on) and change the Boot Order (or Boot Sequence) option to "Floppy", "A:, C:" or similar.
Notice that probably your motherboard will be "dead", since the CIH virus tries to erase its BIOS. If that is your case, read our tutorial on how to recover motherboards "killed" by the CIH virus.