source: http://www.securityfocus.com/bid/11834/info

Microsoft Internet Explorer is reported prone to a vulnerability that may allow a remote site to detect files on the local computer. 

A remote attacker can exploit this issue through the ''sysimage://' protocol handler to detect the existence of a file on the local computer of the Web client viewing a malicious page. This could lead to a disclosure of sensitive information to remote attackers.

<img src="sysimage://C:WINNTNotepad.exe,666"
onLoad="document.write('<b>Cannot Find File!</b>');"
onError="document.write('<b>File Exists!</b>');">