But the software giant has said the technology targeted will not be part of the next version of Windows.
"These reports pose no risk for Microsoft customers," said the firm's Stephen Toulouse in a blog posting.
Vista, previously codenamed Longhorn, is due for release towards the end of 2006, half a decade after Windows XP.
The malicious code affects a Microsoft command shell technology called MSH.
This could replace the simple command shell used in Windows. A command shell lets users enter text-based commands, as in the predecessor to Windows, DOS.
The emergence of the proof-of-concept viruses prompted reports of the first viruses for Vista, as MSH was expected to be part of the new operating system.
But Microsoft has sought to squash talk of Vista viruses. In a posting on the company's security blog, Stephen Toulouse said the MSH technology would not be included in the final version of Vista.
Instead he said that MSH was still being tested and could be included in future versions of Windows developed in three to five years' time.