An "advanced search" form would basically have a bunch of fields like "subject" and "date" and "from" then string them together into a query syntax described below. Then redirect that query to the standard search URL. Implementation would most likely be in PHP. Hard part isn't the programming, it is designing an elegant suer friendly form that matches the query syntax. Actually that isn't so hard either. Hard part is making time to get it done.
http://www.mail-archive.com/faq.html#search .
