Critical Patch

If you are running Windows NT 4.0, Windows 2000, Windows XP or Windows 2003 then continue reading.  If you are running Windows 95, Windows 98, Windows ME or other then this patch won't affect you.  (You should still ensure you have the critical patches in place anyhow.)

Visit Windows Update to download this patch, as well as any other critical patches, and update your system. This patch could be the most serious hole yet found. 

Details are scanty in how the buffer overrun could be exploited but in reading the docs it would appear that anyone who can connect to your Win NT 4/2000/XP/2003 Server system in any fashion could hack in. Or any site on the Internet you visit. Or any software or other control or anything you download when visiting a website.  A firewall will *not* make a difference.

"Because this library is widely used by Windows security subsystems, the vulnerability is exposed through an array of avenues, including Kerberos, NTLMv2 authentication, and applications that make use of certificates (SSL, digitally-signed e-mail, signed ActiveX controls, etc.)"

Vulnerability Note VU#583108
eEye Digital Security Advisory AD20040210
eEye Digital Security Advisory AD20040210-2
Microsoft Security Bulletin MS04-007
Microsoft Knowledge Base Article 252648

Visit Microsoft Security for more general information on how to protect your computer. 

Also make sure your antivirus software is up to date.