Open IIS (Start > Programs > Administrative Tools > Internet Information Services)
Get the properties of your FogBugz application.
If you installed it in your root directory, right click on Default Web Site.
If you installed it as a new virtual directory, right click on that virtual directory.
In the properties dialogue, go to the Directory Security tab, click Edit for Anonymous Access, click Edit again (in some OS's), and make note of the "User name". This is the user account FogBugz uses to acces the web server.