SEATTLE (AP) _ For the second time in eight months, a computer ''virus'' has been found in programs at Seattle software manufacturer Aldus Corp., the company said Thursday.

The company said that despite precautions taken after a virus was discovered in its FreeHand drawing program last March, a new virus has turned up in an updated version of FreeHand.

Further, it said, an entire computer network within Aldus' Seattle headquarters has been infected.

Aldus spokeswoman Laury Bryant said that no programs available for sale to the public were infected by the latest virus.

''We don't know where it came from,'' said Jane Dauber, another Aldus spokeswoman. ''That is the nature of the virus. You can't really track it.''

Viruses are small computer programs, created by programmers as jokes or acts of vandalism, which secretly attach themselves to other programs. The virus can then spread from computer to computer when operators share ''infected'' programs.

At their most benign, viruses merely flash a message on a computer screen. At their worst, they can destroy information stored in computer memories.

Last month, a former Fort Worth, Texas, programmer was convicted of planting a virus in his employer's computer system. Two days after the employee was fired, the virus activated, erasing 168,000 company records.

Aldus officials said their new virus, dubbed ''nVir,'' has remained dormant so far, just a tiny program that attaches itself to other programs..

''We don't know why, we don't know what invokes this virus,'' said Jane Dauber, another Aldus spokeswoman. ''With some of them, you have to launch the program a certain number of times,'' for the virus to activate.

Aldus said it does not know where the virus originated, although the company said the virus apparently has infected at east one East Coast university computer system.

Aldus says it uses rigorous testing procedures to check for viruses in products before they are shipped to retailers. Still, the virus was found in so-called ''beta'' versions of FreeHand software, sent to specific users to be tested before the program is offered for sale.

Aldus said it had shipped beta disks to a dozen businesses and users around the country. One of those users discovered the nVir virus and notified Aldus.

The software company said it took just two days to realize the virus was in its own in-house network of computer syustems. It is not known how far the virus has spread.

''It is in our quality-assurance department,'' said Ms. Dauber, ''and it's unknown at this time how many people had access to that network.''

Aldus is a leading manufacturer of programs for ''desktop publishing,'' in which publications are written, assembled and edited on computers. FreeHand, written for Apple Macintosh computers, allows computer operators to draw graphics.

In March, in what was apparently the first instance of a virus attaching itself to a commercial software program, a virus called the ''March 2 peace message'' was found in some FreeHand programs. At that time, Ms. Bryant said the message did not harm any computers and only flashed a brief message on computer screens. The virus died out of computers on March 2.

Until that time, computer experts had thought viruses could be avoided if users didn't freely exchange software, but only used off-the-shelf programs.

The infection forced Aldus to recall or rework thousands of packages of the new software.

A subcontractor said he inadvertently spread the first virus to Aldus.

Since then, Aldus and other major software companies have started screening software and using programs called ''vaccines,'' designed to weed out viruses.

''We had tested this one for viruses,'' Ms. Dauber said, ''but apparently this is a new virus.''

She said nVir eluded a number of vaccines. It was finally revealed by a new detection program, which was not in use when the beta versions were shipped out.

Spokesmen at other Seattle area software makers, including industry giant Microsoft Corp., said they had experienced no problems with viruses.