DARLINGTON Borough Council computers ground to a halt on Wednesday when its system was hit by the Nachi virus, affecting all its services.

Nachi and Sobig F are two fast spreading viruses which hit computers around the world early last week through infected e-mails.

A council spokesman said its machines began to slow down on Tuesday and were out of action by Wednesday as IT staff worked virtually through the night to cleanse the system.

"It floods your computers, not damaging anything, but rendering everything useless," he said. "Our defences managed to fight off the other viruses but this has seeped through."

"We are working to restore front line services, such as social services, as quickly as we can."

By Thursday departments dealing with housing benefits, council tax and social services were back on line, having been dealt with as a priority.

The Dolphin Centre and Civic Theatre were also back yesterday but experts were still working on the ticket line at the Arts Centre. It was hoped to have Hopetown House, Hunden's Lane and other offices up and running by the end of the week.

The Nachi worm seeks out computers that were affected by the Blaster virus earlier this month and attempts to eliminate that one before spreading to other machines.

The Blaster virus preyed on a vulnerability in Microsoft software and caused machines to perpetually start up or shut down.

Sobig.F infects a computer and sends copies of itself to e-mail addresses stored in the machine. It also installs a Trojan horse program allowing its originator to send e-mail messages through other people's infected computers.