A 32-year-old Greenbelt man pleaded guilty yesterday in federal court here to using his home computer to gain access to administrative files of the Internal Revenue Service and destroying data in the files that was used to test computer programs.

William Van Nest of 7024 Hanover Pkwy., who pleaded guilty to two misdemeanor counts of computer fraud, is the first person convicted in the District of Columbia under a recently passed statute that provides for a maximum year in prison and $100,000 fine for each count.

Assistant U.S. Attorney David F. Geneson told U.S. District Judge Harold H. Greene that the IRS files destroyed by Van Nest contained no information on individual taxpayers or their taxes.

Law enforcement sources said that Van Nest, who formerly worked for a firm that was seeking a contract to provide computer software to the IRS, apparently learned the telephone number that afforded access to the ZILOG computer system while preparing a contract proposal.

The main ZILOG computer is at the IRS' Consulting Services Branch at 5201 Leesburg Pike, near Baileys Crossroads, Geneson said.

On Feb. 26, 1985, he said, an IRS official noticed that files in the ZILOG computer had been destroyed, and that more files were destroyed on March 4 and March 18 of that year.

IRS officials then placed a special monitor on the computer files to try to find out who was destroying the data.

At 2:43 a.m. on March 22, files in the ZILOG computer were again destroyed.

Geneson said that the tracking device determined that a user outside the building was gaining access through the telephone hookup. Most persons who are authorized to use the computer enter a special password and are limited to only certain parts of the data file, Geneson said.

But through the special monitor, IRS inspectors determined that the nighttime invader had used the system's "super user password" -- in this case the word Zeus -- to gain access to all of ZILOG's files and "to trash" some of them, Geneson said.

The invader struck again on March 30, April 2, and April 5.

IRS then set up duplicate files in the backup ZILOG computer in the agency's downtown office and modified the telephone entry to the computer so that any incoming call would go directly to the downtown system.

To make certain that the unauthorized invader knew he was destroying private IRS files, Geneson said, the computer system was changed so that it flashed "authorized access only" as soon as a user gained entry.

At 2:56 a.m. on April 9 and again at 3:17 p.m. on April 11, files were destroyed and a trace of the telephone calls linked them to Van Nest's residence. IRS officials later obtained a warrant and a search of Van Nest's home yielded a computer and sophisticated computer programs, Geneson said.

Van Nest was released on his own recognizance and Greene set sentencing for June 18.