The easy way if you're using J1.5.x is to install NoNumber
Advanced Module Manager it offers lots of ways to apply conditions to modules. If you install
NoNumber Ext. Manager you can install advanced module manager and other plugins and modules from NoNumber using it. It will also give you an easy way to keep upto date with the versions, Peter updates code regularly!
You could also try MetaMod which is another extension which allows conditional control of site content and modules etc. You can check it out
on the JED that link is to the developers page because there are two packages MetaMod and MetaTemplate Pro and Free.