Custom 404 on windows server

I can’t seem to get a .htaccess file to work on a windows IIS6 server for obvious reasons.
I’m only accustomed to linux/apache.

My problem is that when searching my clients site, old links come up that don’t exist.
So I’d like a 404 to re-direct the user to the new home page. Like using the header() function in PHP.

I have no clue how to go about this on windows, plus i’ve heard i may need to use asp.net.
Can someone shed some light?