Joomla upgrade and debug

We need help with a web site which is currently running Joomla 1.0.15 and Virtuemart 1.07. We would like to upgrade to the lastest release of Virtuemart and move the site up to the latest Joomla (1.5.2) plus fix ordering issues. (see below) TELL ME ABOUT YOUR JOOMLA AND PHP EXPERIENCE, and why I should choose you. At some point, (we are not sure when) something went wrong with Virtuemart. Customers now cannot login with their existing accounts, or cannot set up a new account. They are sent to a "You are Not Authorized to View This Page" This needs debugging and fixing. I have set up a separate domain for testing and debugging with a duplicate of the database - it is all the same except there is no secure certificate. There are no customizations or hacks to the code - this is plain Joomla install and plain Virtuemart with a custom template. PM me for details.