Some irresponsible computer hackers are ruining a good thing for everyone. These high-tech vandals are putting booby-trapped software on computerized bulletin board systems.

Users of bulletin boards expect to find innovative and experimental software often not found in software stores. When bulletin board users download a piece of destructive software -- called Trojan horses -- it is likely to erase all the information on a hard disk.

While Trojan horse software has been around since the early days of personal computing, the potential for damage has never been as great as it is now. When hard disks were less common, the worst a Trojan horse could do was erase an entire floppy disk. Now that so many computer users have hard disks and use them for serious business applications, Trojan horses make the idea of using bulletin board software unacceptable.

So, these irresponsible computer hackers have essentially put an end to bulletin board software distribution. Why should anyone risk the possibility of downloading an unknown program if there's a chance it will erase a 20-megabyte or larger hard disk?

Fortunately for those who still want to try to use low-cost and free software not available in stores, there is a way to get it without risk. PC-SIG of Sunnyvale, Calif., is one of the best places in the Silicon Valley to get your hands on public domain software (free) and user-supported software (You're asked to send the author a small fee if you decide to keep using the software.).

Unlike computer bulletin boards, PC-SIG charges a fee for its software. A single disk with several small programs on it costs about $6, and you can get any five of PC-SIG's 700 program disks, and a membership, for about $40.

PC-SIG gets its software from many of the same programmers that use bulletin boards for distribution. But as a business, PC-SIG is more responsible for its software's safety than a bulletin board operator, who has little control over what is sent to the bulletin board while no one is watching.

PC-SIG sells only software that it has tested extensively on its own computers, said Francis Juliano, PC-SIG's technical adviser.

For bulletin board users who refuse to risk downloading software, there are other companies that sell public domain and user-supported software, but PC-SIG is a good place to start.