PITTSBURGH — FLYING high after the successful launch of its new operating system 13 days ago, Microsoft Corporation has run into turbulence from an unexpected direction. Computer viruses are giving the company fits on two fronts. Viruses have tied some users up in knots when they tried to install Microsoft's just-released operating system, Windows 95. Worse, researchers have detected a new kind of virus, native to the company's best-selling word-processing program, Microsoft Word. This new kind of virus is causing serious concern in the computer industry. ''This is a totally new concept of a virus,'' says Jeffrey Leeds, advanced support manager for Symantec Corporation's antivirus group in Cupertino, Calif. ''It's a completely new infection mechanism and that is a big deal,'' adds Richard Ford, a virus expert at the National Computer Security Association in Carlisle, Pa. ''It's going to change the way we think about virus protection a little bit.'' Viruses are rogue computer programs designed to spread. They jump from computer to computer, usually because unwitting users trade diskettes or download electronic files that have been infected. Up to now, viruses could spread only when they attached themselves to files containing computer programs. Now, even data files are suspect. ''In the past, you wouldn't get a virus just by reading your e-mail,'' says Sarah Gordon, a security analyst with Command Software Systems in Jupiter, Fla., which makes an antivirus program. But if the electronic-mail message has a Microsoft Word document attached to it, it is now suspect, she adds. The reason is that Word documents sometimes include more than just data. They can contain small computer programs, known as macros, which automatically execute series of keystrokes. The virus, called Winword.concept, uses the macro language to make it difficult for users to save their documents. According to Microsoft, the virus works only on the latest versions of the word-processing software: Word 6.0 and Word 95. Winword.concept is not very destructive. But it marks the first time that data files have been used to transmit a virus, which may lead to a slew of copycat viruses. ''Word is the first example of this happening, but it's something that can happen to many, many different applications,'' adds Michael Hebert, a group product manager for Microsoft. ''So it is, in a sense, a wake-up call.'' The virus caught the industry so off-guard that Microsoft itself was unwittingly spreading it via an optical, CD-ROM disk it was sending to hardware manufactures so they could test Windows 95. So far, Microsoft reports that less than 0.5 percent of Word users calling for help are reporting problems with the virus. The company this week expects to release new software that will fix the problem, at least temporarily. (Complete instructions are on the company's Internet site on the World Wide Web - http://www. microsoft. com.) Several antivirus companies have also made software available to correct the problem. While Microsoft moves to shore up Word, it is also battling virus problems elsewhere. Last week, the company acknowledged that users may not be able to install Windows 95 if their machine already carries certain viruses. These occasions are rare, apparently, affecting only those users installing the software with diskettes, not CD-ROM disks. According to Microsoft, the viruses generate an error message saying an installation disk is bad. The company recommends using an antivirus program to clean up the machine or, as a temporary measure, flipping the upper left tab on the back of the second disk to keep it from being corrupted. Ironically, antivirus researchers say, Windows 95 offers more protection against viruses than the DOS operating system it replaces.