Virus writers are working on ways of hacking Microsoft's new operating system known as Windows Vista.
Windows software runs on 90% of personal computers worldwide
An Austrian programmer has published examples of malicious code that exploits loopholes in technology which could be part of Vista.
The viruses affect Microsoft's command shell technology which is expected to replace the current command prompt.
Vista, previously codenamed Longhorn, is due for release towards the end of 2006, half a decade after Windows XP.
Finnish security company F-Secure described the malicious code as proof-of-concept viruses to highlight potential vulnerabilities in Vista.
In company's blog, F-Secure's director of anti-virus research, Mikko Hypponen said: "The case is interesting historically, as these are the first viruses for a totally new platform."
The viruses target MSH (Microsoft Command Shell), the technology that Microsoft is looking to use in Vista.
Security experts had warned about the possibility of virus writers exploiting MSH in 2004.
But it is unclear whether the technology will make it into the final version of Vista.
"It has lately been rumoured that MSH might not ship with Vista at all," said Mr Hypponen.
"Instead [it] might be part of Microsoft Exchange 2006 or something. We won't know for sure until later."
Microsoft's flagship Windows software already runs on about 90% of personal computers worldwide.
The firm has promised that Vista will take what for Microsoft is a completely new approach to computing, with security not an add-on but an integral part of the operating system.