Microsoft has released its security patches for September, fixing known vulnerabilities in its MSN Messenger software and Unix services for Windows as well as a critical bug in Windows 2000.

In total, Microsoft patched four bugs in its products. This was one less update than expected following the software maker's last-minute decision to scrap a fix for its SharePoint collaboration software.

The Windows 2000 update is the only one Microsoft rates as critical. It affects the Microsoft Agent software that Web developers use to create interactive characters on Web pages. By tricking a Windows 2000 user into visiting a maliciously encoded Web page, an attacker could exploit this flaw to run unauthorized software on a victim's computer.

This is not the first time Microsoft has been forced to patch a critical bug in Microsoft Agent. In April, the company fixed a similar flaw that also affected Windows XP users.

Because the MSN Messenger and Windows Services for Unix flaws were both publicly disclosed last month, they should also be given priority said Amol Sarwate, manager of Qualys's vulnerability research lab.

"They had both been known for awhile, but they are important," he said. "Especially the MSN vulnerability."

MSN Messenger and Windows Live Messenger users will be prompted to upgrade their software when they connect with Microsoft's instant-messaging services, Microsoft said in its notes on the security update. "If you do not upgrade to a non-affected version of the MSN Messenger or Windows Live Messenger client, depending on your platform, you will be notified to upgrade on each attempt to sign on."

MSN Messenger 7.0.0820 or Windows Live Messenger 8.1 are not vulnerable to the flaw, Microsoft said.

The fourth patch issued Tuesday fixes a bug in Crystal Reports for Visual Studio. Attackers could run code on a victim's PC by getting them to open a specially crafted Crystal Reports RPT file, Microsoft said.

Overall, the September updates are a far less serious group of patches than the nine updates Microsoft released last month.

"Compared to last month, Patch Tuesday in September is almost anticlimactic," security vendor McAfee said in a statement. "Customers who have legacy applications installed on Windows Server 2000 Service Pack 4 should be paying the most attention, but we don't foresee a lot of exploitation of the Windows 2000 vulnerability. Not many people will use those systems to surf the Web, which would be the attack vector."