There is a URL protocol handler command injection vulnerability in 
Internet Explorer for Windows that allows you to execute shell commands 
with arbitrary arguments. This vulnerability can be triggered without 
user interaction simply by visiting a webpage.

When Internet Explorer encounters a reference to content inside a 
registered URL protocol handler scheme it calls ShellExecute with the 
EXE image path and passes the entire request URI without any input 
validation. For the sake of demonstration I have constructed an exploit 
that bounces through Firefox via the FirefoxURL protocol handler. The 
full advisory and a working Proof of Concept exploit can be found at

http://larholm.com/2007/07/10/internet-explorer-0day-exploit/

Cheers
Thor Larholm