Mambo Site Transfer to Joomla

We need to migrate a mambo site to the latest Joomla platform without significant downtime. The problem is.. is that the menu system no longer works in Internet Explorer 7. So, we're taking this opportunity to upgrade the entire site. I believe the Javascript that currently runs the main menu would also need to be upgraded to work with IE7.. while still ensuring that it works with all the other major browsers as well. We'd need to start taking immediate action on this job. I believe it would need to be completed by this week.