http://www.microsoft.com/technet/treeview/default.asp?url=/technet/security/bulletin/MS01-058.asp

Who should read this bulletin: Customers using Microsoft® Internet Explorer. 

Impact of vulnerability: Run code of attacker’s choice. 

Maximum Severity Rating: Critical 

Recommendation: Customers using IE should install the patch immediately. 

Affected Software: 

Microsoft Internet Explorer 5.5 
Microsoft Internet Explorer 6.0