Microsoft could release early a patch for a loophole in its Word program that virus writers are already exploiting.
Users are being warned to watch out for booby-trapped documents
If users get infected by the virus, attackers could open up a backdoor on the PC and take over the machine to use it for their own ends.
A fix is due for release in mid-June but Microsoft said it would bring it forward if the action was "warranted".
Microsoft told users to turn off some Word features in order to protect themselves.
Security firm Symantec said the virus, called Backdoor.Ginwui, that exploits the bug in the word processing software had been found in e-mails bearing a Japanese Word document.
The document summarises a recent summit meeting between the US and Asia. So far only people at one company are known to have been caught out.
Those opening the booby-trapped Word document will fall victim to the virus which opens up a backdoor on the PC. It then reports back to a website in Asia, telling it the computer has been compromised.
Microsoft is planning to release a fix for the flaw on its next monthly patch date which falls on 13 June. But it said on its security advisory blog that the patch may be released sooner "if needed".
To help people protect themselves, Microsoft urged users not to open or save Word files they receive from unknown sources or even those that are from trusted sources but are unexpected.
For extra protection, it said users should change an option in the popular Outlook e-mail program that uses Word as the editor for mail messages.
Other advice included not opening Word files that are inside other programs such as Excel or PowerPoint. It also said users should avoid downloading Word documents from websites.
Users should also run Word in so-called "safe" mode which shuts down some features of the word-processing program and stops the exploit code gaining hold of a machine.
The bug affects Microsoft Word 2002 and 2003 for Windows.