Custom 404 Quesrystring Problem

I've created a custom 404 page that performs redirects based on the incoming
URL cross referenced against a database table.

The problem I have is the way IIS sends on the "not found" information.
Example...

http://www.mysite.com/place/missingpage.asp

is sent on to my custom 404 page by IIS via the Querystring as:

404;http://www.mysite.com/place/missingpage.asp

Which is fine,until I add a Quesrystring to the original URL:

http://www.mysite.com/place/missing...var=10&qwert=11

still comes through as

404;http://www.mysite.com/place/missingpage.asp

without the extra Querystring data from the original URL.

Having researched this online it seems many people have this problem and
looks like IIS 6 solves it. However I've also read of people with IIS 6
having the same issue, and of people with IIS 5 that don't have the problem.
(for exmaple: http://www.webmasterworld.com/forum47/1829.htm)

What I'd like to know is if there is definately no way of achieving this
behaviour on IIS 5 without the use of any extra isapi filters. Perhaps a
server configuration option or something...