There are THREE major variants of W32.CIH.Spacefiller, also known as Chernobyl.

Version 1.2 activates on April 26th.
Version 1.3 activates on June 26th.
Version 1.4 activates on THE 26TH OF EVERY MONTH!!!!

Chernobyl is a DEADLY virus. If infected, Chernobyl WILL ERASE DATA ON YOUR HARD DRIVE, AND MAY STOP YOUR COMPUTER FOM BOOTING UP AT ALL!!

Chernobyl is one of the few viruses known  that ACTUALLY CAUSES DAMAGE to hardware. It's payload was first triggered on April 26, 1999, the 13th anniversary of the Chernobyl nuclear reactor accident in Russia. Rumors run  rampant as to whether Chernobyl was the activation date or whether it was activated a year since it was released into the wild. It was devastating, as it erased the FAT when the infectecd file was executed. 

This is a Windows 95/98 specific parasitic virus infecting Windows PE (Portable Executable), and aboit 1kbyte in length. It targets Win 95/98 machines. It is LOW RISK, but HIGH DAMAGE. Users of Win NT, 2K, and XP are considered immune.   

You have been warned......