The advisory comes in the week before Christmas, a time when many companies and home users are least prepared to deal with the problems. Security firm Symantec warned its clients of the vulnerabilities on Thursday, after the Chinese company that found the flaws published them to the Internet.
One vulnerability, in the operating system's LoadImage function, could enable an attacker to compromise a victim's PC when the computer displays a specially crafted image placed on a Web site or in an e-mail. The other vulnerability, in the Windows Help program, likewise could affect any program that opens a Help file.
Because the flaws are in a library used by Windows programs, almost all browsers and e-mail clients are likely affected by the flaws, said Alfred Huger, senior director of engineering at Symantec.
"They are rather serious," Huger said. "Both can be exploited by anything that processes images or reads help files."
Because the flaws were accompanied by sample code--known as exploit code--that shows how to take advantage of the security holes, Huger expected the exploits to be quickly incorporated into.
"The fact that there is an exploit out there is very concerning," he said. "I think you will see it inin very short order."
A mass-mailing computer virus could also quickly begin using the vulnerabilities to spread.
Microsoft could not immediately be reached for comment on the issues.
The flaws came to light on Thursday, when a Chinese security forum, Xfocus Team, posted the issues to its Web site. The vulnerabilities were found by Chinese firm VenusTech and posted on Monday to the Internet, according to the Xfocus posts.
Software companies and corporate information technology departments are often short-staffed during the holiday season. That could mean that the response to this latest threat will be slow, Huger said.
"It is a bad time of year for this to come out," he said.