The fast-spreading virus which has been wreaking havoc with computers is not thought to have caused any more mayhem.
The Sobig.f virus, is still being sent by email at a rate of around one million copies a day.
It was programmed to direct PCs to download files from special websites between 8 and 11pm on Friday.
But Carole Theriault, a consultant at anti-virus firm Sophos, said that 19 of the 20 sites had been blocked off.
"Only one was left active, and nothing seems to have happened there," she said.
It is not known what the downloading would do, but possibilities included launching another virus or spam attack, or deleting files stored on an infected computer or network.
Ms Theriault said: "It's really good news. However there is a similar danger time between 8pm and 11pm on Sunday night."
She noted that the virus, which has been clogging inboxes with unwanted spam messages, was showing no signs of abating.
Users who think they may have an email with the virus are urged not to double-click on the attachment. Companies are advised to install the latest anti-virus software, and to disinfect infected PCs.