Server Density or Scout-like Services for Windows?

Server Density has a Windows monitoring agent now, so I'd still recommend it.

Don't know what the size of your Windows install-base is, but if you want to go "whole hog" the Microsoft monitoring system is System Center Operations Manager 2007 R2. It's not exactly cheap compared to other solutions out there, but it basically has every conceivable monitoring and alerting option covered.

They almost have it completely scriptable via PowerShell.

However, it can be a real pain to get configured. You are probably better off sticking with the product you already understand/use in a "lean" team.