Importing XML database from a hacked-together custom CMS

I run a blog that’s been going for quite a few years now, and I am looking to convert to WordPress. Right now I have a 90 MB XML file containing thousands and thousands of posts, tags, comments, etc.

What would be the easiest and quickest way to import all this data into WordPress? Is there something like this available? Or could I somehow import the MySQL database directly, choosing which fields map to which?

Or am I going to have to figure out how to manipulate XML with C/Java?