As if things weren't tough enough already in the slumping residential real estate market, Northern Virginia real estate agents for the last week have had to deal with the four most dreaded words of the electronic age: The system is down.

Approximately 200 angry agents, representing thousands who were recently cut off from the computerized listings of Virginia homes for sale, yesterday aired their complaints in an emotional meeting with top officials of the Northern Virginia Association of Realtors. Last January, the Realtors association signed a five-year contract with Dator Corp. of Greenwich, Conn., for a new computerized multiple listing service (MLS).

The MLS is intended to maximize exposure of a seller's residential property by providing a master list to which all agents in Northern Virginia have computerized access. According to many agents, however, the new MLS, known as React, has had exactly the opposite effect since it was activated July 1.

"For one solid week, I watched that system degrade to the point where I couldn't even get into it," said Bob Cohen, an associate broker in the Annandale office of Re/Max Central Realtors. "Friday night, I was totally fed up, so I decided to do something about it."

Last Saturday morning, Cohen faxed a flier to Northern Virginia real estate offices asking agents to participate in yesterday's protest at the Realtors association's Fairfax headquarters. Given yesterday's turnout, Cohen's fax apparently captured the frustration that had been sweeping the community of real estate agents.

According to Renee Miller, president of the Realtors association, the organization needed a new system because the incumbent MLS provider, McLean-based Planning Research Corp., had told the board two years ago that it might not be able to service its expanding MLS. Miller said the 17,000-member Northern Virginia board is the biggest in the world and that the conversion of its MLS is the largest in history.

"The PRC system was working well," Miller said. "But once PRC told the board that it was concerned, the five members of our MLS felt a responsibility to look for other vendors to see which one could provide the system our group wanted."

Through a selection process in which Miller said agents participated in a series of meetings, Dator, a privately held 40-employee company that specializes in converting MLS systems, won the contract. The company previously had converted the MLS on Long Island, N.Y., which Dator said is the country's second-largest MLS. According to Miller, that achievement helped Dator win the contract.

There is a debate over whether the new Dator system for Northern Virginia has flawed software, making it impossible to use, or whether brokers just aren't familiar enough with it. But it is access to listings, not explanations, that agents want.

Without the MLS, "I can't give the kind of service my clients are paying for," said Phyllis Hoag, an associate broker in the Alexandria office of Long & Foster Realtors, who attended yesterday's meeting.

"I've had to keep three listings off the market because I can't get into the computer to list them on the MLS," Hoag said. "The system is totally inoperative. It's been that way for the last week. If I want to show clients property, I have to drive around and look for 'For Sale' signs. My clients are very understanding, but they want to sell their property. All I can tell them is to wait a couple of weeks. I feel totally helpless."

Hoag's pain may be eased shortly. A committee of 10 agents, including Cohen, has been organized to convey the agents' concerns to the Realtors association. According to Cohen, the committee's first action will be to recommend reprogramming the React software so that agents can obtain MLS information through the same computer techniques they used on the old system.

"People being upset is the natural course of events when you implement an MLS of this size," said Dator President Michael Jacobson. "People's expectations are not realistic if they expect that there wouldn't be problems with a conversion of this size. The soundness of the software is not in question. Realistically, the system should be stabilized for those who know how to use it in the next day or two. For those people still behind the training curve, they're just going to need more experience using the system."

"The general consensus is that agents are into real estate, not computers," Cohen said. "I'm not looking to pin responsibility on anybody. I just want the problem corrected so we can have the tools to do our job."