The updates, released Tuesday, fix nine bugs in Microsoft's products. In addition to the four critical updates, Microsoft also released "important" fixes for the Windows SharePoint collaboration software and in the Windows remote procedure call (RPC) technology.

Nine updates is one less than originally expected. Last Thursday, Microsoft said it was planning to fix an unnamed flaw in Windows 2000 and Windows Server 2003 that could be used for "spoofing." That update was not included in Tuesday's patches.

Microsoft pulled this Windows patch because of a "quality control issue," the company said in a statement. It's not unheard of for Microsoft to make such last-minute decisions. The SharePoint update that was released Tuesday had previously been pulled from the September updates for similar reasons.

The Word vulnerability may be of particular concern.

That's because it has been exploited by attackers, according to Christopher Budd, security program manager with Microsoft's Security Response Center. "We're aware of very limited and targeted attacks, but the issue itself has not been publicly disclosed," he said.

The bug can be exploited when an attacker tricks a victim into opening a maliciously crafted Word document. It was reported to Microsoft sometime during the past three months, Budd said. This type of attack has been used many times over the past few years, exploiting bugs in a variety of Microsoft Office products. These attacks have been launched at a small number of select victims, often within government.

Though the Windows RPC flaw apparently cannot be exploited to run unauthorized software on a victim's machine, security experts consider it one of the most important patches released Tuesday. RPC service has been the source of many virulent computer worms in the past, including 2003's Blaster, according to Amol Sarwate, manager of Qualys' vulnerability research lab.

"Whenever there is a denial of service there is always a chance of remote code execution," he said. However, even if hackers could find a way to use this bug to run unauthorized software on a PC, RPC is typically blocked at the firewall. This means that even if a worm could be crafted, it would have difficulty spreading.

Sarwate said that the IE patch, which fixes four bugs in the browser, should be moved to the front of the line by system administrators. That's because IE is so widely used and some of the bugs in the browser are very likely to be used in online attacks.