Windows runs most of the world's computers and has been a constant target of attacks by worms, trojans, viruses and other malicious software. Microsoft routinely releases security updates and patches.
However, it was unusual for the US government's anti-terrorism department to call on the public to seal a crack in Windows before attackers broke in.
"This vulnerability could impact government systems, private industry and critical infrastructure, as well as individual and home users," the DHS said in a release.
Attackers could take advantage of the software weakness to remotely take control of computers, according to the DHS.
PC users were called on to apply Microsoft security patch MS06-040 "as quickly as possible."
The department said its Computer Emergency Readiness Team was working with Microsoft to protect or minimize the damage to government machines.
Attempts to exploit vulnerabilities in operating systems typically spike in the days after security patches are released.
Computer security professionals advise people to keep their operating system software current and use up-to-date anti-virus programs.
Click This To Download MS06-040