Locked out of all instances of Geoserver due to password problem

I beginning to deploy Geoserver (2.6.1), Tomcat 7 on Windows Server 2008 and everything was going just fine until I changed the master password but I now find that I am completely locked out of my instance since my password is not now recognised (not sure why, possible initial mistyping). I decided to start again so uninstalled Java, Tomcat + Geoserver but with a fresh install of Java, Tomcat and Geoserver I remain locked out of any instance of Geoserver whether the default admin/geoserver or my password. I changed the user.xml to admin/geoserver as others have advised, read the security documentation, went through the whole uninstall process again but I’m still locked out. I can only assume that the master password must be buried very deep somewhere in the operating system and re-surfaces for any new instance. Hoping I don't have to re-install the server OS! Any help appreciated.