Q:  What is http://www.msftncsi.com/ncsi.txt and why is it in my proxy logs?!

A: Windows Vista includes a feature called Network Connectivity Status Indicator (NCSI).  For more information see, "Appendix K Network Connectivity Status Indicator and Resulting Internet Communication in Windows" and a snippet is below:

NCSI might communicate with a Web site to determine whether a network has Internet connectivity.  A page called ncsi.txt (http://www.msftncsi.com/ncsi.txt) containing the following line of text with no terminating new line or other non-printing characters: "Microsoft NCSI".  NCSI can be disabled by changing a registry setting.