Poxy proxy – keep getting prompted for user credentials in IE

Recently while working on a legacy installation that was being virtualised some issues with a proxy server emerged. The one in question was WebMarshal 2006 and the symptom was that out of 20 users one user was continually prompted for their domain username and password.  The infrastructure was SBS 2003 with Isa 2004 and the client was xp sp3 with Ie 8.

The user could not browse the web at all due to this problem. Testing options included moving user to another machine (worked) – creating new user on machine (worked) – domain admin on machine (worked). So pretty sure the issue lay with user config – tried creating new profile but still unable to access web.

On a hunch I downloaded Safari for windows with another machine  and installed on to the problem machine – initial page load failed but when the user tested another page from one of Safaris shortcuts they were prompted for proxy username / password and once supplied they were surfing again.

Haven’t go to the bottom of the IE issue but suspect it relates to something cached on the machine under the users specific login / registry that prevents it successfully authenticating with the proxy. More about that once I find the source but at least the user can browse and work with Safari.