The Mydoom e-mail worm, considered by internet security experts to be the "fastest spreading e-mail worm in history," was primed to attack SCO's website and experts say it will move on to target Microsoft Corporation on Tuesday.
"A large scale denial of service attack has started that has made the company's website www.sco.com completely unavailable," SCO said in a statement.
SCO, which owns the UNIX operating system, said that by midnight (0500 GMT) on Sunday its website was flooded with requests beyond its capacity.
"This large scale attack, caused by the Mydoom computer virus that is estimated to have infected hundreds of thousands of computers around the world, is now overwhelming the Internet with requests to www.sco.com," said Jeff Carlon, SCO's worldwide director of Information Technology infrastructure.
"We have a series of contingency plans to deal with this problem and we will begin communicating those plans on Monday morning," Carlon said.
The variant of the worm attacking SCO, Mydoom.A, is set to become ineffective on 12 February.
"This is the biggest single Distributed Denial of Service (DDoS) attack ever"
anti-virus firm, Finland
Computer security experts said the Mydoom attack was the biggest they had witnessed.
"This is the biggest single Distributed Denial of Service (DDoS) attack ever," the Finnish anti-virus firm F-Secure said.
"We estimate the total amount of infected computers to be over one million," F-Secure said. The security group had described Mydoom as the "fastest spreading e-mail worm in history."
Experts fear the super bug could receive a boost on Monday morning as thousands of computer users, particularly in the United States, switch on their office computers after the weekend break.
The Mydoom worm has left hundreds of thousands of computers vulnerable to hackers, spammers and other cyberspace outlaws and its economic fallout has been estimated at $26.1 billion so far, according to the British security firm mi2g.
Meanwhile, Microsoft is bracing for an attack by a variant of the virus called Mydoom.B, which is due to target Microsoft's corporate website on Tuesday at 1309 GMT.
Mydoom.B is set to be deactivated on 1 March.
The hunt for the creators of the virus has so far proved fruitless although suspicion is centering on Russia, where Mydoom first appeared.
Microsoft and SCO have offered a $500,000 reward for information leading to the arrest and prosecution of Mydoom's authors.