The owners of tens of thousands of bot-infested PCs in the county -- who've resisted calls all week to update or install anti-virus software -- will likely switch on their PCs on Friday to find their data gone, said computer security specialist AhnLab.

From midnight local time (3 p.m. GMT Thursday) the virus, which has been attacking prominent U.S. and South Korean government and commercial Web sites all week, has been programmed to encrypt user data or reformat the hard drive of the PC.

There are still ways to save an infected PC, although if the owners have ignored security requests so far they might be unlikely to follow AhnLab's recommendations. These involve starting Windows in safe-mode by using the boot menu accessed through the F8 key at start-up, setting the clock to before July 10 and then rebooting the PC normally and updating anti-virus software or performing a free scan to erase the virus.

The attacks have been headline news all week in South Korea, where casualties have included the top-ranked news Web site, one of the leading online auction sites, electronic banking portals of several major banks and the home pages of the Ministry of National Defense, the president's Web site, the National Assembly and the U.S. Forces Korea.

Computer security companies have been urging people to update their ant-virus software or download an application to perform a free scan but many have, apparently, ignored those requests.

A third wave of attacks on Thursday night overloaded some of South Korea's most popular Web sites and showed that the bot-infested PC army was still alive and kicking.

But Thursday night's attacks might be the last. This shift from attack to destroy may indicate the end of this particular round of attacks, which started on July 4 against U.S. sites and hit South Korean sites for three days in a row this week.

Little is known about the person or persons controlling the virus although computer security experts say the attack itself is not particularly sophisticated. That leaves the possible range of culprits wide, from individuals with a relatively low level of hacking skills to organized groups or governments who might have employed a low-tech approach to confuse experts.