I was attempting to install Microsoft Operations Manager 2005 on my laptop a week or so ago, and struggling to work out why the MOM installed kept saying that the pre-requisites for installing MOM weren't up to scratch. Specificially, it was complaining that SQL Server 2000 SP3 or later was not installed. I knew it was, in fact I had SP4 installed. Fortunately, the answer was already out on the Internet. You need to "tweak" the registry temporarily during the installation to make MOM think that SQL 2000 SP3 is installed, and then change it back again afterwards. Annoying.
Here's Russ Kaufmann's explanation of the steps you need to follow.