WASHINGTON -- Most of NASA's computer programs were back to normal Friday, a day after the so-called "love-bug" computer virus forced the space agency for the first time to shut down its e-mail system at centers around the country.

Only the sprawling Johnson Space Center (JSC) in Houston remained out of touch electronically -- its e-mail clobbered by the virus that appeared in messages such as "ILOVEYOU."

Thursday's outage marked the first time NASA has been without e-mail since the mid-1980s, when the space agency began installing an internal electronic-messaging system.

"We got hit at multiple centers so, yeah, it was pretty serious," said Brian Dunbar, a spokesman at NASA Headquarters, which assessed the damage at its 10 field centers.

"But no mission-critical systems were affected. If something was lost, it was on people's [computer-screen] desktops," he said.

JSC also took the unusual step of closing down its website for security purposes while technicians installed new anti-virus software and systems.

Until everyone is back online, "well go back to the old-fashioned method of phones, faxes and talking to people," said Kelly Humphries, a JSC spokesman.

JSC, which is the home of the astronauts and Mission Control for both the space shuttle and the International Space Station, uses the type of computer software -- Microsoft Outlook -- that the virus had targeted.

It could remain without e-mail through the weekend, Dunbar said.

"Obviously the people who use the Microsoft suite of products are going to be more affected," Dunbar said. "And JSC relies on Microsoft products, more so than the other centers which went in under one contract for their systems."

Other centers that lost e-mail capability on Thursday were the Kennedy Space Center in Florida, the Goddard Space Flight Center in Greenbelt, Maryland and the Marshall Space Flight Center in Huntsville, Alabama.

Those centers were back to normal by Friday afternoon.

But even as most of their computer systems came online, security experts at NASA and elsewhere were battling two new strains spawned from original virus -- considered the worst the world has seen.

The subject line of one read "VERY FUNNY JOKE" while the other was headlined "MOTHER'S DAY."

Computer experts said original "love" virus was far more devastating than last year's "Melissa" bug. The "love bug" racked up more than $1 billion in losses worldwide and infected tens of millions of computers around the world.

The bug, which was carried in an e-mail attachment on Microsoft Outlook, wipes out certain photo and multimedia files when opened. It overwrites those files with a garbled version and then passes the virus on to all addresses contained in a user's Outlook address file.

"The phone still works and the fax machine still works so things will get done. If someone is used to sending documents as a computer attachment, they'll just have to get a hard copy and walk it down the hallway."

Even without the e-mail, though, the space agency continued to function.

"The phone still works and the fax machine still works so things will get done," Dunbar said. "If someone is used to sending documents as a computer attachment, they'll just have to get a hard copy and walk it down the hallway."

Also bitten by the "love bug" were NASA Headquarters, the Glenn Research Center in Cleveland, Ohio, the Stennis Space Center in Mississippi and the Ames Research Center in Mountain View, California.

At the Kennedy Space Center, where NASA's $8 billion fleet of space shuttles is readied for flight, quick action enabled the center to avoid a migraine of major proportions.

"We were able to catch the bug early on and prevent it from spreading," said Bruce Buckingham, a KSC spokesman. "Damage appears to be minor and recoverable."

In California, home to three NASA centers, the bug's effects were minimal.

"No operations were impacted -- none," said John Bluck, a spokesman at Ames. "A very small number of desktops were affected."

At the Jet Propulsion Laboratory in Pasadena, California, workers posted photocopied alerts around the leafy campus, warning against any e-mails with "ILOVEYOU" in the subject line.

"We caught it in time," said Nancy Lovato, a JPL spokeswoman.

The bug also did not appear to have much of an effect on the nation's military space program.

The Pentagon said it had encountered the virus on some of its computers Thursday but that no classified systems were affected.

The virus also reached into computers at the Central Intelligence Agency in McLean, Virginia, which uses data from classified spy satellites, but the effect was "negligible," a spokesman said.