Malicious hackers are already known to be exploiting the flaw via booby-trapped and compromised websites.
Microsoft usually issues security patches once a month to help users keep their PC safe.
However, the seriousness of the bug has prompted the software company to act early and stifle attempts to exploit the flaw.
Cursor cure
The problem started to receive public attention in late March when security firms realised that the way Windows handles animated cursors could be used as a route to take over a PC.
Microsoft said it had decided to issue a patch early because attacks using the vulnerability had increased in intensity and code to exploit the flaw was known to be circulating widely.
McAfee warned that attackers could booby-trap websites with the exploit code and "silently" compromise vulnerable PCs.
On its Security Response Center blog Microsoft said it had been notified about the flaw in December 2006 and had been working on a fix since then.
The fix was scheduled to be released on 10 April - the next date for Microsoft's regular monthly security update.
"Due to the increased risk to customers from these latest attacks, we were able to expedite our testing to ensure an update is ready for broad distribution sooner than April 10," noted the blog.
PC users will be able to get the fix via Windows automatic update or visit Microsoft itself to download the patch manually.
Users of Windows Vista, XP, 2000 and Server 2003 are potentially vulnerable to the cursor vulnerability.
OneCare was the only failure among 17 anti-virus programs tested by the AV Comparatives organisation.
Microsoft's software only spotted 82.4% of the 500,000 viruses that the independent group subjected it to.
The test is the second in less than a month that Microsoft's anti-virus software has failed.
Minimum standard
Live OneCare is Microsoft's flagship security program that, like many other anti-virus products, is designed to help PC users keep their machine clear of malicious software.
Austria-based AV Comparatives carries out quarterly tests of top anti-virus programs to find out if they have kept up with the growing mass of viruses circulating online.
The tests see if anti-virus software can spot a variety of malicious programs including macro and script viruses, worms, backdoors and trojans.
The majority of programs tested, 14, got an advanced pass or better from AV Comparatives; two got a standard rating and OneCare failed.
OneCare fared particularly badly against so-called polymorphic viruses which regularly change their configuration in a bid to fool security software.
"Microsoft OneCare performed very low in the test, and did not reach the minimum requirements for participation," wrote Andreas Clementi, senior tester at AV Comparatives, in the report about the tests.
A Microsoft spokesman said: " We are looking closely at the methodology and results of the test to ensure that Windows Live OneCare performs better in future tests."
He added that Microsoft was trying to: "determine whether any learnings from these tests can be used to improve our services as part of our ongoing work to continually enhance Windows Live OneCare to ensure the highest level of protection and service that we can provide our customers."
The spokesman also pointed out that Windows OneCare had been certified by ICSA Labs and the West Point Checkpoint system.
In early February, security magazine Virus Bulletin revealed that OneCare failed to spot all the viruses it tested the software against.